ENH: openfoam shell session - improved and relocated

- '-c' option (as per shell), '-Dkey[=value]' option to provide
  preferences via the command-line. For example,

      etc/openfoam -DWM_COMPILER=Clang -int64  ./Allwmake -j -s -l

  These can also be combined with other options. Eg,

      etc/openfoam -DWM_COMPILER=Clang \
          -c 'wmake -show-path-cxx -show-cxxflags'

- relocated from bin/tools/ => etc/ for easier access

- bin/tools/openfoam.in : for autoconfig-style installation

- Auto-detect if the shell script was executed with openfoam and
  interpret accordingly.

  Simple example,

      --------------
      #!/usr/bin/openfoam
      cd "${0%/*}" || exit   # Run -*-sh-*- from this dir

      blockMesh
      simpleFoam
      --------------

   Note it is NOT currently possible to provide any other parameters
   this way. Eg,

      `#!/usr/bin/openfoam -sp` (NOT)

   This will either fail to run, or result in infinite recursion.