usnic: handle missing ibv_event_type_str

Some older versions of libibverbs do not have `ibv_event_type_str`,
leading to compilation failures on older machines, irrespective of
whether they could ever support usNIC anyway.  If we encounter any other
build issues related to "old verbs" then we should just cause the usnic
BTL to disqualify itself when it encounters "old" traits.

Thanks to Paul Hargrove for reporting the issue:
http://www.open-mpi.org/community/lists/devel/2014/02/14056.php

Reviewed-by: Jeff Squyres <jsquyres@cisco.com>

cmr=v1.7.5:reviewer=ompi-rm1.7
Provide better messages when we don't find any included interfaces, and/or don't find any interfaces for use by OOB.

cmr=v1.7.5:reviewer=jsquyres