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.

changed milestone to %v2012

added enhancement label

changed the description

Example with current openmpi-system (without version)

ld: warning: libmpi.so.12, needed by ../linux64GccDPInt32Opt/lib/openmpi-system/libPstream.so

after upgrading to leap15.2 and openmpi2 as system openmpi.

approved this merge request

all looks good - please confirm all ok when using dash

added 6 commits

• 1d2da6c5...67b3c01e - 4 commits from branch develop
• 26ec9b5a - CONFIG: rationalize compiler tuning
• c91fc6f4 - CONFIG: rationalize mpi config tuning (#1910 (closed))

Compare with previous version

looks okay with dash (ubuntu 20.10)

mentioned in commit cf6d5b84

merged