Convert the list of syscache names from a series of #define's into an enum,

Convert the list of syscache names from a series of #define's into an enum,
to avoid the pain of manually renumbering them anytime we insert another
name in alphabetical order.  An excellent idea from Alex Hunsaker and
NikhilS' inherited-constraints patch --- whether or not the rest of that
gets in, this should.  Dunno why we never thought of it before.