BUG: missing foamExec causes foamJob failure (#1309, #1411)

- foamExec was removed as part of commit f308aa61 but
  overlooked the fact that it is partly still needed by foamJob.
  When running in parallel, it is used to source the etc/bashrc env.

  Reinstated a stripped down form, which has been relocated to
  bin/tools directory (it should not be directly called by the user).

  The previous version switching functionality has nonetheless be
  dropped since it only worked only when a rigid naming convention for
  OpenFOAM installations was followed.

ENH: add foamJob log=, -log-app, -no-log options, improved coding quality