Warning of incompatible change to string functions

[Moon at MIT-AI (David A. Moon)]

In system 51, STRING-REVERSE-SEARCH-SET and STRING-REVERSE-SEARCH-NOT-SET
will do what the manual has always documented them as doing.  Currently they
return 1+ what they are documented to (except when they return NIL).  I have
fixed all callers of these functions in the system (the two outside the file
LISPM2;STRING thought they worked the way the manual says anyway!).  If any
users happen to call these they will need to adjust their code accordingly.