Fix rvalue for socket::send() on EHOSTUNREACH
Fix rvalue for socket::send() on EHOSTUNREACH

During introduction of EHOSTUNREACH, missing mapping between
EHOSTUNREACH errno and false/0 return code for socket's send()
calls was missing, throwing an exception.

To be consistent with the rest of wrappers (e.g. DONTWAIT), fix it
by handling this errno as a regular EAGAIN, and let the caller use
errno/zmq_errno() to branch on their code.

zeromq/cppzmq,zeromq/cppzmq