Expose ZMQ_MAX_SOCKETS_DFLT as a constant

Expose ZMQ_MAX_SOCKETS_DFLT as ZMQ::CTXOPT_MAX_SOCKETS_DEFAULT.

Additionally, update the 032-contextopt.phpt test case so that it won't
fail if ZMQ_MAX_SOCKETS_DFLT changes.

Fixes mkoppanen/php-zmq#118