fix warn unused variables

Provide scatter/gather array I/O

We have to use an incomplete type in the interface.
The definition of iovec is only in the implementation.
This appears to following existing practice in 0MQ.
It seems a bit flakey that zmq.h is not included in zmq.cpp,
which is normal practice to ensure the implementation matches
the specified interface. YMMV. I follow 0MQ style.

Also move iovec definition from implementation to interface.
Not clear to me at present if an incomplete type should
be used to avoid gratuitously including <sys/uio.h> in
the interface. The interface can't be used with this include.

Posix style send multiple messages using scatter/gather array.

sys/uio.h contains the XSI vector I/O data structure.

Fix comments to conform to style guide.

Thread Safe Sockets.

Semicolon missing

add missing semicolon

We use a distinct context initialisation function to specify
all sockets derived therefrom will be thread safe.

However the inheritance is done exclusively in the C interface.
This is not really correct, but it is chosen to minimise
interference with the existing C++ code, including any
construct or other calls within the C++ code base.
Semantically the C++ code should be unchanged,
physically some data structures and extra methods are
provided by they're only used from the C binding.

1. Reorganise C API socket functions to eliminate bad practice
of public functions calling other public functions. This should
be done for msg's too but hasn't been in this patch.

2. Reorganise code in C API socket functions so that the
socket is cast on one line, the C++ function called on
the next with the result retained, then the result is returned.

This makes the code much simpler to read and also allows
pre- and post- call hooks to be inserted easily.

3. Insert pre- and post- call hooks which set and release
a mutex iff the thread_safe flag is on.

4. Add the thread_safe_flag to base_socket_t initialised to
false to preserve existing semantics. Add an accessor for
the flag, add a mutex, and add lock and unlock functions.

Note: as yet no code to actually set the flag.

LIBZMQ-300: free received subscription messages

mato: merge patch to close libzmq-268

mato: merge patch to close libzmq-303

mato: merge patch to close libzmq-205

With the introduction of subscription forwarding, the first message sent
on a PUB socket using a unidirectional transport (e.g. PGM) is always
lost due to the "subscribe to all" being done asynchronously.

This patch fixes the problem and also refactors the code to have a single
point where the "subscribe to all" is performed.
Signed-off-by: Martin Lucina <martin@lucina.net>

Reduce memory usage of mtrie.

Signed-off-by: Staffan Gimåker <staffan@spotify.com>

Support Epoll on cross-compile without runtime test.

Updated connector open() doc strings.

Fix for LIBZMQ-305