common/verbs: update usnic transport probe

RHEL 7 has shipped with kernel support for the RDMA_TRANSPORT_USNIC
enum, but ''not'' the RDMA_TRANSPORT_USNIC_UDP enum.  This means that
when you install usNIC drivers from cisco.com, the kernel will report
IBV_TRANSPORT_USNIC, even though the transport is actually using UDP.

Therefore, we have to modify the logic in common/verbs to do the
additional magic probe if the device reports either an
IBV_TRANSPORT_IWARP or IBV_TRANSPORT_USNIC (because both of those might
be lies -- do the probe to figure out the real transport).

The code changed by this patch is fairly trivial; it simply moves the
logic of the magic probe to its own short function, and then calls that
short function in both the IBV_TRANSPORT_(IWARP|USNIC) cases.  It looks
longer because several lengthy comments were also updated.

Authored-by: Jeff Squyres <jsquyres@cisco.com>
Reviewed-by: Dave Goodell <dgoodell@cisco.com>

cmr=v1.8.2:reviewer=ompi-rm1.8