Move the fake usnic IBV provider out of common/verbs and into a new
common/verbs_usnic component that is always statically linked into
libopen-pal. The fake provider is registered with libibverbs at run
time, but there is no *un*register IBV API. Hence, we can't let the
code containing this provider be dlclosed -- which means it needs to
be statically linked into libopen-pal.
Fixesopen-mpi/ompi#1060.
