ENH: relocate WM_PROJECT_SITE default (issue #1050)

- was PREFIX/site, now PROJECT/site

  This avoids several issues when installing OpenFOAM in clusters
  without an intermediate OpenFOAM-specific installation prefix.

  The 'site' directory may have a reserved meaning in these situations
  and it is undesirable to 'leak' upwards into the parent directory to
  look for configuration files.

  Placing the default within the project directory avoids this.
  Alternative locations can be given via the WM_PROJECT_SITE variable.

etc/config.csh/settings

@@ -154,7 +154,7 @@ setenv FOAM_EXT_LIBBIN $WM_THIRD_PARTY_DIR/platforms/$WM_ARCH$WM_COMPILER$WM_PRE
 if ( $?WM_PROJECT_SITE ) then
     set siteDir=$WM_PROJECT_SITE
 else
-    set siteDir=$WM_PROJECT_INST_DIR/site
+    set siteDir=$WM_PROJECT_DIR/site
 endif
 
 # Shared site (group) executables/libraries
@@ -177,7 +177,6 @@ endif
 if ( -d "$siteDir/$WM_PROJECT_VERSION/bin" ) then   # Version-specific
     _foamAddPath "$siteDir/$WM_PROJECT_VERSION/bin"
 endif
-unset siteDir
 
 _foamAddPath ${FOAM_USER_APPBIN}:${FOAM_SITE_APPBIN}:${FOAM_APPBIN}
 _foamAddLib  $FOAM_LIBBIN/dummy     # Dummy versions of external libraries last
@@ -333,7 +332,7 @@ endsw
 
 # Cleanup
 # ~~~~~~~
-unset archDir
+unset archDir siteDir
 unset gcc_version gccDir
 unset gmp_version gmpDir  mpfr_version mpfrDir  mpc_version mpcDir
 unset clang_version clangDir