This commit adds support for placing the send memory segment in a
traditional shared memory segment when XPMEM is not available. The
current default is to reserve 4MB for shared memory on each process.
The latest benchmarks show vader performing better than sm on both
Intel and AMD CPUs.
For large messages vader will now use CMA if it is available (and
XPMEM is not).
cmr=v1.7.5:reviewer=jsquyres
This commit was SVN r30123.
