[Nmh-workers] Changes to mhbuild

[Ken Hornstein]

Greetings all,

I've committed the following changes, which affect mhbuild and a few other
programs:

- mhbuild now defaults to a Content-Transfer-Encoding of 8bit for text
  parts.
- mhbuild now has a -maxunencoded switch to control the automatic selection
  of quoted-printable as a CTE (if a text part contains a line longer than
  -maxunencoded, it will be encoded with quoted-printable rather than 7bit
  or 8bit).
- mhbuild directives can now select the Content-Transfer-Encoding (see
  the man page for details)
- The MIME encoder/decoder routines now correctly encode and decode Base 64
  encoded text parts (before the line endings were not properly handled).

Feedback is welcome.

--Ken