Add day-of-week prefix to string returned by FILE-TIME->STRING. At

Add day-of-week prefix to string returned by FILE-TIME->STRING.  At
this point, the returned string is fully compliant with RFC 822.
Furthermore, the procedure is not operating-system specific, except
that it calls the procedure LOCAL-TIME-ZONE, which isn't currently
defined for any other operating system.  Offhand, it's not obvious
what's involved in implementing LOCAL-TIME-ZONE for other systems,
except for the more modern unix systems, in which it's trivial.