ENH: additional text expansion shortcuts (issue #792)

Support the following expansions when they occur at the start of a
string:

    Short-form       Equivalent
    =========       ===========
      <etc>/          ~OpenFOAM/   (as per foamEtcFile)
      <case>/         $FOAM_CASE/
      <constant>/     $FOAM_CASE/constant/
      <system>/       $FOAM_CASE/system/

These can be used in fileName expansions to improve clarity and reduce
some typing

     "<constant>/reactions"   vs  "$FOAM_CASE/constant/reactions"