Support the IBV_*_USNIC_* verbs constants.

These constants are now upstream (see
https://git.kernel.org/cgit/libs/infiniband/libibverbs.git/commit/?id=f57a9c67eabb9e7f19c624ac3c8c27b7be55796c),
so let's support them properly in Open MPI.

Added bonus: consolidating these checks up in
ompi_check_openfabrics.m4 allowed removing some custom checks and
AC_DEFINE's from the usnic configure.m4 script.

Also change the usnic/configure.m4 check for IBV_EVENT_GID_CHANGE to
use AC_CHECK_DECLS (vs. AC_CHECK_DECL).

cmr=v1.7.5:reviewer=dgoodell