changes required to support MPI_Bsend(). Introduces concept of
attaching a buffer to a large segment that the PML can scribble into and
we will send from. The reason we don't use a pinned buffer and send
directly from that is that usnic_verbs does not (yes) support num_sge>1
for regular sends. This means the data gets copied twice, but that is
unavoidable.
changed the logic in handle_large_send to be more sensible
Incorporated David's review comments
This commit was SVN r29184.