Rationalised WM_PROJECT_SITE and WM_PROJECT_INST_DIR in foamEtcFile and etcFiles

for consistency with WM_PROJECT.  Now "etc" files are assumed to be in etc
sub-directories of WM_PROJECT_SITE and WM_PROJECT_INST_DIR allowing other files
to be stored in those directories.  The search order is now:

Search for files from user/group/shipped directories.
The search scheme allows for version-specific and
version-independent files using the following hierarchy:
- \b user settings:
  - ~/.OpenFOAM/\<VERSION\>/
  - ~/.OpenFOAM/
- \b group (site) settings (when $WM_PROJECT_SITE is set):
  - $WM_PROJECT_SITE/\<VERSION\>/etc/
  - $WM_PROJECT_SITE/etc/
- \b group (site) settings (when $WM_PROJECT_SITE is not set):
  - $WM_PROJECT_INST_DIR/site/\<VERSION\>/etc/
  - $WM_PROJECT_INST_DIR/site/etc/
- \b other (shipped) settings:
  - $WM_PROJECT_DIR/etc/

\return The list of full paths of all the matching files or
an empty list if the name cannot be found.
Optionally abort if the file cannot be found.
Optionally stop search after the first file has been found.

This change was proposed and agreed by the sponsors of the OpenFOAM project on
the OpenFOAM Hub, see https://openfoam.org/maintenance/

doc/codingStyleGuide.org

@@ -115,9 +115,9 @@
         //    - ~/.OpenFOAM/
         //      <em>for version-independent files</em>
         //  -# site-wide settings:
-        //    - $WM_PROJECT_INST_DIR/site/\<VERSION\>
+        //    - $WM_PROJECT_INST_DIR/site/\<VERSION\>/etc/
         //      <em>for version-specific files</em>
-        //    - $WM_PROJECT_INST_DIR/site/
+        //    - $WM_PROJECT_INST_DIR/site/etc/
         //      <em>for version-independent files</em>
         //  -# shipped settings:
         //    - $WM_PROJECT_DIR/etc/