SystemInformation: support for resource limits

Add methods to report host memory total, host memory available,
process memory available, host memory used, and process memory
used. In this context memory is unavailable if there are resource
limits in place that would prevent its use. Such resource limits
assumed to be applied on a per host basis both to cooperatively
operating process groups, such as mpi programms running in parallel,
and to individual processes. When reporting host memory available
consult an application specified environment variable. When reporting
process memory available consult unix resource rlimits and an
application specified environment variable. The environmant variables
provide a means of communicating resource limits that are being
applied in a non-standard way.

Change-Id: Ifb3b0fdaab8db0ab87140fa2dcafad3c51e2d874