CONFIG: rationalize mpi config tuning (#1910)

- prefix FOAM_MPI and library directories with 'sys-' for system
  versions for uniform identication.

  WM_MPLIB      | libdir (FOAM_MPI)  | old naming |
  SYSTEMMPI     | sys-mpi            | mpi        |
  SYSTEMOPENMPI | sys-openmpi        | openmpi-system |

- prefix user preferences with 'prefs.' to make them more easily
  identifiable, and update bin/tools/create-mpi-config accordingly

      Old name: config.{csh,sh}/openmpi
      New name: config.{csh,sh}/prefs.openmpi

  Retain support for config.{csh,sh}/mpi-user since this is relied
  upon in spack packaging.

- additional mpi preferences now available:
    * prefs.intelmpi
    * prefs.mpich
    * prefs.mpich-gm
    * prefs.msmpi
    * prefs.mvapich2
    * prefs.sys-mpi

CONFIG: added hook for EASYBUILDMPI, similar idea to USERMPI

ENH: support different major versions for system openmpi

- for example, with

     WM_MPLIB=SYSTEMOPENMPI2

  defines FOAM_MPI=sys-openmpi2 and thus creates lib/sys-openmpi2

ENH: centralize handling of mpi as 'mpi-rules'

    Before:
        sinclude $(GENERAL_RULES)/mplib$(WM_MPLIB)
        sinclude $(DEFAULT_RULES)/mplib$(WM_MPLIB)

        ifeq (,$(FOAM_MPI_LIBBIN))
            FOAM_MPI_LIBBIN := $(FOAM_LIBBIN)/$(FOAM_MPI)
        endif

    After:
        include $(GENERAL_RULES)/mpi-rules

- also allows variants such as SYSTEMOPENMPI2 to be handled separately