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.