- avoid editing of settings.{sh,csh}

- this change was wished by Mattijs and/or Henry

- allow command-line settings to retain the flexibility of 'foamPV'
  alias mechanism.

This reverts commit 2e1e671d.

- drop FOAM_MPI_LIBBIN variable in favour of FOAM_MPI from which
  two different locations can be determined.

  Previously there was something like this:

      FOAM_MPI_LIBBIN = $FOAM_LIBBIN/openmpi-1.5.1

  We now instead determine the locations from the FOAM_MPI directly:

      FOAM_MPI=openmpi-1.5.1

          $FOAM_LIBBIN/$FOAM_MPI     -> OpenFOAM  mpi-related
          $FOAM_EXT_LIBBIN/$FOAM_MPI -> ThirdPary mpi-related

ENH: relocate ParaView libraries and plugins back under FOAM_LIBBIN

- as discussed with Mattijs

- avoid non-standard 'tempfile' in bin/tools/foamConfigurePaths

- eg,
  . PATH_TO_FOAM/etc/bashrc myPrefs.sh

etc/cshrc: allow option setting for option software to be specified in ~/OpenFOAM/???/apps/<application>/cshrc

- this provides a means to temporarily override the default settings
  and/or the prefs.{csh,sh} settings.

  bash:
      name=       -> unset name
      name=value  -> export name=value

  csh:
      name=       -> unsetenv name
      name=value  -> setenv name value

  eg,
      . <WM_PROJECT_DIR>/etc/bashrc WM_COMPILER=Gcc45 WM_MPLIB=
  to use gcc 4.5 and the default mplib

Otherwise all $PATH will pick up the local one which will not be synchronised with $path

- the path is accesible either way, but use setenv for consistency with
  other environment variables. It also simplifies foamCleanPath again.

BUG: compilerInstall was getting overwritten in settings.{csh,sh}

editing the respective settings files.

- foamEtcFile is used to find the files, thus follows the standard
  ~OpenFOAM/fileName expansion mechanism.