First commit of the Cisco usNIC BTL.

This BTL accesses the Cisco usNIC Linux device via the Linux verbs
API via Unreliable Datagram queue pairs.  A few noteworthy points:

 * This BTL does most of its own fragmentation; it tells the PML that
   it has a very high max_send_size (much higher than the network
   MTU).
 * Since UD fragments are, by definition, unreliable, the usnic BTL
   handles all of its own reliability via a sliding window approach
   using the opal_hotel construct and many tricks stolen from the
   corpus of knowledge surrounding efficient TCP.
 * There is a fun PML latency-metric based optimization for NUMA
   awareness of short messages.
 * Note that this is ''not'' a generic UD verbs BTL; it is specific to
   the Cisco usNIC device.
Add some notes about the Cisco usNIC BTL.
Remove Elan and Windows Verbs from the list of supported networks.