[Nmh-workers] minor change to repl -convertargs

[David Levine]

When processing directives from repl -convertargs, mhbuild used to
inhibit use of quoted-printable for text content.  I just removed
that.   Now, the encoding is selected based on content, just as it
is for all other message content.  The user still has control over
the use of quoted-printable via the mhbuild -maxunencoded switch.
Though now, lines over 998 bytes long will always be encoded.

David