Feature rationalize mpi configs
The configuration files, user preferences and wmake rules for MPI had become rather confusing and cumbersome. This is especially noticeable when trying to add in minor differences for handling system-level openmpi2 vs openmpit4 etc. Using a single directory name 'openmpi-system' makes it difficult (impossible) to properly handle different major openmpi versions on the same system.
And additional problem was posed by the choice of preference names. An 'etc/config.sh/mpi' handles all MPI-related dispatch, whereas etc/config.sh/openmpi
handles ThirdParty adjustments only. Thus make preferences clearly marked with a "prefs." prefix.
Reorganized MPI make rules for more commonality.
Incompatibility name change for advanced user:
- "openmpi-system" -> prefs.sys-openmpi
- "openmpi" -> prefs.openmpi
Prefix system-related mpi with sys-
, for sorting and consistency.