ENH: support search modes for etcFiles()

- similar to the foamEtcFile script -mode=... option, the specific
  search location (user/group/other) can now also specified for
  string expansions and as a numerical value for etcFile()

  For example, if searching for group or other (project) controlDict,
  but not wishing to see the user controlDict:

    1.  foamEtcFile -mode=go controlDict

    2.  fileName dictFile("<etc:go>/controlDict");
        dictFile.expand();

    3.  etcFile(controlDict, false, 0077);

  The default behaviour for searching all contexts is unchanged.

    1.  foamEtcFile controlDict

    2.  fileName dictFile("<etc>/controlDict");
        dictFile.expand();

    3.  etcFile(controlDict);