usnic: certain send frag members are immutable

Ensure that they never are touched by checking in their destructors.

Reviewed-by: Jeff Squyres <jsquyres@cisco.com>
Reviewed-by: Reese Faucette <rfaucett@cisco.com>
usnic: add MSGDEBUG1_OUT/MSGDEBUG2_OUT macros

This includes suppressing picky-mode warnings about __VA_ARGS__, which
we know are supported by any compilers we care about.

Reviewed-by: Jeff Squyres <jsquyres@cisco.com>
Reviewed-by: Reese Faucette <rfaucett@cisco.com>
usnic: refactor callback invocation

This makes it a little easier to see what's happening with callbacks to
the PML.

Reviewed-by: Jeff Squyres <jsquyres@cisco.com>
Reviewed-by: Reese Faucette <rfaucett@cisco.com>
usnic: pack via convertor on the fly

If we need to use a convertor, go back to stashing that convertor in the
frag and populating segments "on the fly" (in
ompi_btl_usnic_module_progress_sends).  Previously we would pack into a
chain of chunk segments at prepare_src time, unnecessarily consuming
additional memory.

Reviewed-by: Jeff Squyres <jsquyres@cisco.com>
Reviewed-by: Reese Faucette <rfaucett@cisco.com>
usnic: fix segfault at finalize time

Without this commit, if you run IMB pingpong between two nodes with only
one usnic selected (e.g., via `--mca btl_usnic_if_include usnic_0`) then
the run will seem fine but will segfault at MPI_Finalize time.

This behavior has happened since Cisco v1.6 git commit ec7ddf8, upstream
trunk r29484, and upstream v1.7 r29507.

Root cause was that the free list element was being used as the recv
buffer instead of the data buffer associated with the element.  So the
reassembly code would stomp all over the free list element, which would
cause the destructor to explode when the free list attempted to clean up
all of its elements.  This surprisingly did not cause any other problems
until now.

Reviewed-by: Reese Faucette <rfaucett@cisco.com>