CONFIG: improve prefix matching for system libraries (#1607)

- missed detection of system libraries when installed with multiarch
  paths like /usr/lib/x86_64-linux-gnu

CONFIG: improve handling of group/user config files (#928)

- changed bashrc handling of FOAM_CONFIG_NOUSER to use
  FOAM_CONFIG_MODE instead. Propagate into foamEtcFile to make this
  a stickier control.

  This change allows better control, but also enables cluster
  installations to define their own value within the OpenFOAM prefs.sh
  file to prevent users accidentally mis-configuring things if
  necessary.

- remove undocumented handling of an (a)ll mode in foamEtcFile to
  avoid potential pitfalls.

- add support for FOAM_CONFIG_ETC handling.
  This allows injection of an extra search layer when finding
  project etc files

ENH: improvements to foamConfigurePaths (#928)

- handle FOAM_CONFIG_ETC implicitly, or explicitly with the new
  -etc option.

STYLE: more explicit wording in foamConfigurePaths usage (#1602)

- document that an absolute path (eg, -scotch-path) overrides/ignores
  the equivalent ThirdParty setting (eg, -scotch)

- longer options -system-compiler and -third-compiler for -system
  and -third, respectively. Clearer as to their purpose.

- adjust the location sanity check to look for META-INFO directory.

etc/cshrc

@@ -35,10 +35,16 @@
 #       source /path/etc/cshrc WM_COMPILER=Clang WM_LABEL_SIZE=64
 #
 # Environment
-#     FOAM_CONFIG_NOUSER (set/unset)
-#         - suppress use of user/group configuration files
+#     FOAM_CONFIG_ETC
+#     - alternative/additional location for OpenFOAM etc/ directory
+#
+#     FOAM_CONFIG_MODE (search mode for etc config files - see foamEtcFile)
+#         - eg, FOAM_CONFIG_MODE="o" to only use OpenFOAM config files
+#         - no influence on OpenFOAM applications, just the config files
+#
 #     FOAM_VERBOSE (set/unset)
 #         - add extra verbosity when sourcing files
+#
 #     WM_PROJECT_SITE (optional directory)
 #         - local site-specific directory, uses WM_PROJECT_DIR/site if unset
 #