ENH: better handling of mpich for openfoam packages

- define MPICH_SKIP_MPICXX, link libmpich instead of libmpi
  (as per disussion in PR #4990)

ENH: more flexible editing/patching of openfoam env files

CONFIG: comment out requirement for openmpi+thread_multiple

- was originally slated for the 1712 release and beyond, but there are
  reportedly unstable version/network combinations.
  Leave as comment for future reference, since it may be arise again.

STYLE: renamed openfoam-site.patch as 1706-site.patch (and reformatted)

- to preserve against impending changes in the layout of
  config.{csh,sh}/settings