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

- contains the name part of the FOAM_CASE environment variable