Abstract the SPAM_LIST as a generic REPLACE_LIST

Abstract the SPAM_LIST as a generic REPLACE_LIST

REPLACE_LIST can be used more generally as a list of pattern
match-replace settings.  SPAM_LIST was a special case of this, so
spam handling has been been changed to use REPLACE_LIST instead, and
SPAM_LIST was removed.

A generic function for performing a REPLACE_LIST replacement has
been added in mutt_apply_replace().

Commited by Kevin McCarthy with some buffer overflow fixes in
mutt_apply_replace().