usnic: convert from verbs to libfabric (yay!)

This commit represents the conversion of the usnic BTL from verbs to
libfabric.

For the moment, libfabric is embedded in Open MPI (currently in the
usnic BTL).  This is because the libfabric API is still changing, and
also has not yet been released.  Ultimately, this embedded copy of
libfabric will likely disappear and the usnic BTL will rely on an
external installation of libfabric.

New configure options:

* --with-libfabric: will cause configure to fail if libfabric support
    cannot be built
* --without-libfabric: will prevent libfabric support from being built
* --with-libfabric=DIR: use an external libfabric installation
* --with-libfabric-libdir=LIBDIR: when paired with --with-libfabric=DIR,
    use LIBDIR for the libfabric installation library dir

The --with-libnl3[-libdir] arguments are now gone.
common verbs: remove usnic-specific code

Now that the usnic BTL uses libfabric, we can remove the
usnic-specific code from opal/mca/common/verbs.
.gitignore: ignore usnic unit test executable
libfabric: sync to upstream libfabric github

Bring down the latest from the libfabric github, as of
9d051567c8eb7adc2af89516f94c7d0539152948.
usnic: update to latest libfabric API changes
usnic: fix show_help message

Rename a few symbols to use libfabric-friendly names.  Fix a show_help
message when fi_av_insert times out.
usnic: remove SVN-style $Id$ tokens (and #idents)

This commit is also upstream in libfabric.
libfabric: various configury updates

1. Ensure to override CFLAGS properly.  Move the setting of CFLAGS outside the AM_CONDITIONAL so that Automake doesn't get confused (because CFLAGS is already set inside an AM_CONDITIONAL -- moving it outside the conditional ensure that this local CFLAGS override trumps all other CFLAGS overrides).
2. Only build libfabric on Linux. Add a little more configury to ensure that we only try to build libfabric on Linux.
3. Remove a dead/unused file
4. Fix typo in condition check
5. Use "false", not "/bin/false"
psm_am.h: add missing file from libfabric snapshot

This is just about to be fixed upstream, but "make dist" was not
including this file in the libfabric tarball.