A bug fix and an enhancement to mhfixmsg based on patches

provided by M. Levinson:
1) Set status to NOTOK in get_multipart_boundary() if the
   message is missing its end boundary.  This avoids a seg fault
   in fix_boundary(); also added check for null return from
   parse_mime().  If this turns out to be a common problem, we
   could consider adding code to repair it.  Now, mhfixmsg ignores
   the condition and proceeeds with any other transformations.
2) With -decodetext 7bit, mhfixmsg now converts base64-encoded
   parts to quoted-printable 8bit if the part will not fit into
   7bit.