Solution: if a PING message contains a context, echo it back in the
PONG message. In order to do so, create the PONG message when PING
is received and store it in the engine.
After the PING the engine goes straight to encoding and sending, so
there can always be at most one pending PING.
Add tests for various contexts.

Solution: treat the first byte of the command body as the size of the
command name, rather than as an id, to comply with ZMTP 3.1.
This was not an actual problem at runtime since both heartbeat
commands have a size of 4, which was treated like an id.
But once SUBSCRIBE/UNSUBSCRIBE get implemented it needs to be checked.

Solution: remove it, it doesn't add much value anyway

Solution: migrate to unity

Problem: building on Linux with Sun Studio still fails

Solution: initialise it

Solution: add compiler flags to support extensions like anonymous
structs in unions, fix casting of const, and check for suncc rather
than solaris

Problem: no way to specify source interface for UDP multicast sender …

Solution: if a binding interface is provided in the sender URL we pass
it to IP[V6]_MULTICAST_IF setsockopt

Problem: test_radio_dish triggers aliasing errors with some compilers

Solution: refactor the test code to work around aliasing issues

Problem: build broken on Solaris with Sun Studio compilers

Solution: change the ifdef to always print on any Unix, as they
all have ulimit

Solution: use a different max socket value on Solaris, where the
default limit is 256 instead of 1024

Solution: add missing const qualifier to internal functions

Solution: do it explicitly to fix build on Solaris 10/11 with the Sun
compiler

Solution: remove constness on pair arguments. Fixes #3090

Problem: warnings in MSVC builds around size_t/int

Solution: use proper types

Problem: getrandom test does not check if it's working

Problem: ZMQ doesn't expose the MULTICAST_LOOP socket option

Solution: update to document new binding options and IPv6 support

Solution: add a new ZMQ_MULTICAST_LOOP option for UDP sockets.

Problem: test helper recv_string_expect_success ignores flags

Solution: pass the flags to zmq_recv

UDP Ipv6 support

Solution: Add IPv6 support

Solution: check return value in autoconf and CMake. On some platforms
the function is available but not implemented (eg: GNU/Hurd).

Add ip_resolver.cpp and ip_resolver.hpp for vs2015 libzmq project

acinclude.m4: check if -latomic is needed.

On some cases, -latomic is needed for likning, and since the current
acinclude.m4 checks only compilation we can sometimes miss the need for -latomic
and the linking process will fail.
Therefore, the AC_CHECK_IFELSE was replaced with AC_LINK_IFELSE. If the first
try fails, we try to link again with -latomic and add LIBS="-latmoic" in case we
succeeded.
Signed-off-by: Asaf Kahlon <asafka7@gmail.com>

Mcast bind

for multicast

Solution: augment the UDP URL syntax to accept an interface specifier with a
syntax similar to the PGM urls.

Fixes #2212

Solution: factor the code into ip_resolver, add IPv6 support and unit tests.

Udp parser

Solution: replace it with the ip_resolver code shared with the TCP
address code

It simplifies the UDP parsing code and makes it behave more like the
TCP counterpart, in particular it's not possible to connect to hosts
by name and bind by NIC names.

It also adds support for "*" port resolving to 0 (useful to let the OS
allocate the port number).

Solution: add unit tests

Problem: ip_resolver allows wildcard ports for non-bindable sockets

Solution: return an error in this situation but still allow using an explicit
"0" if somebody really wants to connect to port 0.

This shouldn't break any existing code because a "*" port was already rejected
in an early test in the TCP path in zmq::socket_base_t::connect.