CONFIG: improve flexibility of MPI specification

- it is now possible to include the selected mpi version in the
  top-level bashrc or prefs.sh file. For example,

      WM_MPLIB=OPENMPI-4.1.1  or  WM_MPLIB=openmpi-4.1.1

  after evaluation of the config.sh/mpi, this will define

      WM_MPLIB=OPENMPI-4.1.1  and  FOAM_MPI=openmpi-4.1.1

  During the wmake, the mpi-rules will first load the MPI 'family'
  rules (OPENMPI in this example) before trying to load
  version-specific rules if they exist.

  NOTE: the regular user-defined prefs system is unaffected by this
  change. This means it is still possible to use a file such as
  'prefs.openmpi' to define the preferred version instead or as well.
  However, it does mean inconsistent naming can be specified.
  For example, specify WM_MPLIB=OPENMPI-4.1.1 at the top-level but
  actually have FOAM_MPI=openmpi-4.0.6 in the prefs.openmpi file.
  This will make the value of WM_MPLIB misleading.

CONFIG: foamConfigurePaths support for sys-openmpi major version

CONFIG: cleanup any shadow env variables