Solution: ensure message short nonces are strictly increasing and validate them

Solution: accept only the mechanism defined by the socket options.

I've not tested this yet, so it's a speculative fix.

For OS X, the microseconds field is implemented as an int type. The implicit narrowing in the initializer list throws a compiler error for some compilers with C++11 support turned on. The specific error message is: "error: non-constant-expression cannot be narrowed from type 'long' to '__darwin_suseconds_t' (aka 'int') in initializer list [-Wc++11-narrowing]".

Tested on Clang 5.1.0 and Mac OS X 10.9.4.

When Curve authentication is used, libsodium opens a file
descriptor to /dev/urandom to generate random bytes. When
the ZMQ context terminates, it should ensure that file gets
closed.

It's bad practice to start by testing all exceptional conditions
and then dropping through to the 'normal' condition. Apart from
being inefficient, it's deceptive to the user. Conditional code
should always try to show the natural expectation of the code,
with exceptional cases coming last.

Solution: clean up this code.

- not initialized before use
- name is nasty (boolean variables should not have negative names)

Solution: rename to 'lossy' and initialize to 'true'.

Solution: change setsockopts on printable keys to expect 41, nor 40
bytes. Code still accepts 40 bytes for compatibility, and copies the
key to a well-terminated string before using it.

Fixes #1148

Updated:
    src/thread.cpp: On older z/OS UNIX System Services,
        pthread_{get,set}schedparam is not present (searching the
        Internet suggests it may be present in later version than
        the one being used for z/OS UNIX System Services porting).

        Make zmq::thread_t::setSchedulingParameters() a no-op on
        z/OS UNIX System Services.

    NOTE: pthread_{get,set}schedparam appear to have been introduced
    by POSIX.1-2001 or IEEE 1003.1-2004 so may not be universally
    available, and thus more platforms may need this "no-op" treatment.

Updated:
    src/metdata.hpp: Remove explicit "const" from key of std::map<>
       because the key is implicitly const already (see
       http://en.cppreference.com/w/cpp/container/map and
       http://www.cplusplus.com/reference/map/map/).

On some platforms (such as z/OS UNIX System Services) explicitly
declaring the map key as "const" causes template expansion errors
as it tries to create separate allocators for "const const std::string"
and "const std::string" only to find that they clash.  (Presumably
some compilers collapse these into one earlier.)

There are no template expansion errors if the map key is left to be
implicitly const.

Updated:
    src/signaler.cpp: Add close_wait_ms() static function to loop
       when receiving EAGAIN in response to close(), with ms long
       sleeps, up to a maximum limit (default 2000ms == 2 seconds);
       used in signaler_t::~signaler_t() destructor.

It must be done this way, as WSAGetLastError returns 0 in these circumstances

- new code may help undersdtand issue #1108
  (https://github.com/zeromq/libzmq/issues/1108)
- code cleanups

environment variables.

The fix should be sane on all UNIX-like systems, so there's
no ZMQ_HAVE_FREEBSD involved.  It's likely that other BSDs
stumble across this problem too.

Solution: set errno to EINVAL when a property does not exist.

Also fixed test_metadata.cpp to test this case.

This reverts commit 78a7b469.

Solution: set errno to EINVAL when tcp:// endpoint is invalid (was just
leaving errno to previous value).

Rationale: In a real-time environment it is sometime mandatory to tune
threads priority and scheduling policy. This is required by our users
who mixes real-time and server threads within
the same process. It's not planned to support this on non-pthread
platforms (e.g. Windows).

This syntax is undocumented and has no known meaning. It was in libzmq
since 3.x.

Solution: remove this code.

Since https://github.com/zeromq/libzmq/commit/350a1a, TCP addresses
get resolved asynchronously, so zmq_connect no longer returned an
error on incorrect addresses.

This is troublesome since we rely on some error checking to catch
blatant errors.

Solution add some upfront syntax checking that catches at least the
obvious kinds of errors (invalid characters, wrong or missing port
number).