- this improves the chances of having a current version

- doc/THIRD_PARTY lists where various third party sources can be found

- still needs more fleshing out, but introduce the method for now and
  add more code later (without recompiling everything else).

- use shift-right instead of shift-left formulation to avoid wrong behaviour
  with non-optimized compilation when the packed items fit exactly in the
  available number of bits.

- Locate user/site/shipped file with the semantics used in the
  ~OpenFOAM/fileName expansion

This reverts commit 82e59fa1.

- useful location for sharing in-house solvers, libraries, etc
  for use at the entire site

- similar naming convention as ~OpenFOAM expansion:

  FOAM_SITE_APPBIN=
  $WM_PROJECT_INST_DIR/site/$WM_PROJECT_VERSION/bin/$WM_OPTIONS

  FOAM_SITE_LIBBIN=
  $WM_PROJECT_INST_DIR/site/$WM_PROJECT_VERSION/lib/$WM_OPTIONS

- avoid ambiguities caused by relative paths