Added a new "boolean" type to mh.h and TRUE and FALSE constants.

Added a new "boolean" type to mh.h and TRUE and FALSE constants.

Added a note to DIFFERENCES stating that it's out-of-date (Richard was the last
one to update it) and that we should consider only documenting incompatibilities
with MH there.

Implemented (and documented) a third kind of username masquerading: "plussed
user" masquerading.  This one was suggested by Neil Rickert
<rickert@cs.niu.edu>.  It's based on sendmail's "plussed user" feature, where
mail sent to <user>+<string> will be delivered to <user>.  When it's enabled,
it's controlled by the $USERPLUS environment variable.  How is it enabled?
Well, that leads me to:

Renamed the "mmailid:" setting in mts.conf to "masquerade:", and changed it so
that rather than being a boolean, it can be set to any combination of the three
values "draft_from", "mmailid", and "plussed_user".  Thus it is now possible to
enable the three types of masquerading individually.

Fixed a bug with "mmailid" masquerading (dating back to MH?)  where if it was
turned on, ','s would no longer be considered GECOS field delimiters.