Convert mutt_expand_fmt() and rfc1524_expand_filename() to struct Buffer

Convert mutt_expand_fmt() and rfc1524_expand_filename() to struct Buffer

These two functions were tied together and so are converted at the
same time.

Note that rfc1524_expand_filename() had an off-by-one error for the
left hand size of the name template.  It was only copying i-1 instead
of the i characters before %s.

Co-authored-by: Richard Russon <rich@flatcap.org>