Added militant API checking on zmq_getsockopt
Problem: when libgssapi isn't available, GSSAPI options 'seem' to work

Solution: use same approach as for libsodium/CURVE, i.e. return EINVAL
if the library isn't present when libzmq builds, and the application
still tries to use these options in zmq_getsockopt/setsockopt.
Problem: need way to probe library capabilities

As libzmq is compiled with optional transports and security mechanisms,
there is no clean way for applications to determine what capabilities
are actually available in a given libzmq instance.

Solution: provide an API specifically for capability reporting. The
zmq_has () method is meant to be open ended. It accepts a string so
that we can add arbitrary capabilities without breaking existing
applications.

zmq.h also defines ZMQ_HAS_CAPABILITIES when this method is provided.
Trivial whitespace fixup
Fixed compile error if Kerberos installed
Merge pull request #1097 from hintjens/master

Added capabilities API