btl/vader: improved single-copy support

This commit makes the folowing changes:

 - Add support for the knem single-copy mechanism. Initially vader will only
   support the synchronous copy mode. Asynchronous copy support may be added
   int the future.

 - Improve Linux cross memory attach (CMA) when using restrictive ptrace
   settings. This will allow Open MPI to use CMA without modifying the system
   settings to support ptrace attach (see /etc/sysctl.d/10-ptrace.conf).

 - Allow runtime selection of the single copy mechanism. The default behavior
   is to use the best available. The priority list of single-copy mehanisms is
   as follows: xpmem, cma, and knem.

 - Allow disabling support for kernel-assisted single copy.

 - Some tuning and bug fixes.