BACKPORT: spack configuration patches (as per OpenFOAM-v1706)

etc/bashrc
  - improved robustness when sourcing
  - source top-level prefs.sh first (for sysadmin changes)

etc/config.*/settings
  - write job control information to the user directory
  - site/ directory under the OpenFOAM project dir, not its parent dir

etc/config.*/mpi
  - added USERMPI as place for spack mpi information

mgridgen, zoltan:
  - make location configurable

metis, scotch:
  - also check lib path (not just lib64)