Expose ZMQ_MAX_SOCKETS_DFLT as a constant

From the commit message:

> 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.

This is one way of fixing #118.