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"
