http://www.open-mpi.org/community/lists/devel/2009/08/6618.php
lower the default priority of PML/cm to allow _defined_ behaviour
for systems, where both MTLs and BTLs are available (Portals and MX).
Keep the previous behaviour of favoring in case of PSM.
Still, the user may select --mca pml cm for apps where applicable.
This commit was SVN r21834.