This completes support for the PATSUBST builtin function.  It also changes
the matching to be case-sensitive, like MMS.  If you want a case-insensitive
match (well, more like SUBST) then use the :: substiution suffix.
This set of changes introduces support for the SUBST builtin function.  Like
PATSUBST, this uses a case-sensitive match.  Use the :: substiution suffix
for case-insensitive matching.  This should be mentioned in the documentation.