Solution: try to resolve the TCP endpoint passed by the user in the
zmq_unbind call before giving up, if it doesn't match.
This fixes a breakage in the API, where after a call to
zmq_bind(s, "tcp://127.0.0.1:9999") with IPv6 enabled on s would
result in the call to zmq_unbind(s, "tcp://127.0.0.1:9999") failing.
Add more test cases to increase coverage on all combinations of TCP
endpoints.

Problem:
Conditional logic in check_protocol() that checks if a protocol is supported,
is duplicated twice. Moreover, the first set of checks to ascertain if a
protocol is supported is done regardless of whether the particular protocol
will be built into the library or not.

Solution:
* Simplify/collapse all supported protocol checks into one in check_protocol()
* Enclose pgm/epgm/norm socket+protocol match checks with requisite macros

Solution: set errno to ENOCOMPATPROTO in check_protocol()

Solution: make the same change for the ZMQ_POLL_BASED_ON_SELECT case

Solution: correct it to spaces

Solution: check for GCC version before using pragma diagnostic
in src/tweetnacl.c to avoid an additional warning.

Solution: return -1 (no event) instead of 0 (event)

For some reason, this just returns 0 if there are no sockets registered
on the poller. Usually this would mean there has been an event. So the
caller would have to check the return value AND the event, or write code
that takes the number of registered sockets into consideration.

By returning -1 and setting errno = ETIMEDOUT like in the usual timeout
cases, it's more consistent and convenient.

Test case included.

Solution: use zmq_assert to ensure it's not a nullpointer

Solution: if options.use_fd do not create temporary random
directory for ipc://*, since the socket is already created and
passed to the library by the user.

Solution: check for availability in autoconf and cmake, and if not
available fall back to random file name rather than random directory.

Solution: use the less nice but correct int constant 1000000000
instead of the shorter 1E9 to avoid a compiler warning when assigning
to timespec.tv_nsec, which is a long int.

Solution: in the Windows-specific ifdef in tcp_listener set_address,
check for error and set errno only after the IPv4 fallback has failed
too, to avoid setting errno when the socket creation succeeds through
the fallback.

Solution: if opening an IPv6 TCP socket fails because IPv6 is not
available, try to open an IPv4 socket instead when creating and
connecting a TCP endpoint.

Solution: if opening an IPv6 TCP socket fails because IPv6 is not
available, try to open an IPv4 socket instead when creating and
binding a TCP endpoint.

Defining thread priority for SCHED_OTHER is implementation defined.
Some platforms like NetBSD cannot reassign it as they are dynamic.

<http://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_getschedparam.html>

Credit goes to <kamil@netbsd.org> and <riastradh@netbsd.org> for finding this
solution.

Problem: Since pull request #1730 was merged, protocol for REQ socket is
checked at the session level and this check does not take into account
the possibility of a request_id being part of the message. Thus the option
ZMQ_REQ_CORRELATE would no longer work.
This is now fixed: the possiblity of a 4 bytes integer being present
before the delimiter frame is taken into account (whether or not this
breaks the REQ/REP RFC is another issue).

Ref. https://github.com/zeromq/libzmq/issues/1419

A Visual Studio build from master (commit id: dac5b45d) using the v140_xp toolset yields a binary that is not XP compatible.

Two libraries contain exports that cannot be found:
 -  IPHLPAPI.DLL : if_nametoindex
 - KERNEL32.DLL : InitializeConditionVariable

The latter export is already dealt with in the file './src/condition_variable.hpp'; however this requires setting the _WIN32_WINNT pre-processor definition.
I am not experienced enough to figure a work around for the 'if_nametoindex' method, so I have created a new pre-processor definition 'ZMQ_HAVE_WINDOWS_TARGET_XP' and removed the calling of the function with the limitation that these builds cannot handle a IPv6 address with an adapter name.

To make it easier for people targeting XP with an MSVC build I have modified the MSBuild property file to add/modify the pre-processor definitions if they are building using a XP targeting tool set; such as v140_xp.

I changed this to ZMQ_USE_SODIUM to be consistent with other
configuration options (especially ZMQ_USE_TWEETNACL).

Solution: fix it.

Solution: use ZMQ_USE_LIBSODIUM to match ZMQ_USE_TWEETNACL

Solution: use proper preprocessor macro

libsodium calls abort() when /dev/urandom can't be found
even if one creates ZeroMQ context before calling chroot()[1].

This happens because crypto gets initialized on handshake,
and at that moment the process is already chroot'ed.

Solution: initialize cryptographic libraries in ctx

randombytes_close() is already there in the destructor.

[1] https://download.libsodium.org/doc/usage/index.html

Problem: when using ZMQ_REQ_RELAXED + ZMQ_REQ_CORRELATE and two 'send' are
executed in a row and no server is available at the time of the sends,
then the internal request_id used to identify messages gets corrupted and
the two messages end up with the same request_id. The correlation no
longer works in that case and you may end up with the wrong message.

Solution: make a copy of the request_id instance member before sending it
down the pipe.

Updated handling of Unix Domain Sockets, make use of temporary directories, and cleanup afterward.  Fix test_term_endpoint handling of optvallen

Solution:
* Check gssapi/gssapi_generic.h header file, it is not in Heimdal.
* Check libgssapi too, libgssapi_krb5 is not separated in Heimdal.

Libsodium has started returning -1 in some cases.

Solution: allow and handle error returns from these calls.

Fixes #1831

Platforms that have struct ucred, typically declare it in sys/ucred.h

Solution: socket_base_t::in_event cannot do anything useful with
return status of process_commands. Asserting is the wrong solution,
as it is entirely valid to be interrupted or for the context to be
terminated, so discard the value.

Solution: use NULL instead to avoid build failure with older
compilers.

Solution: initialize class variable in the same order as they are
defined.