Change MAKE-MAIL-BUFFER to return the buffer it makes. If an old

Change MAKE-MAIL-BUFFER to return the buffer it makes.  If an old
buffer exists and the user wants to keep it, return #F instead.  Also
make the SELECTOR argument optional, and allow it to be #F meaning
that the buffer will not be selected but just returned (the default).
Change the MAIL-SETUP-HOOK variable so that it is invoked with the
mail buffer as an argument.