rfc2047.el needs sanitizing

[Dave Love]

I made some quick fixes to rfc2047.el for Gnus 5.9, but it's never
been sorted out properly to avoid runtime depenencies it shouldn't
have on other Gnus files.  That's relevant as it's used in sendmail.el
now.  I suspect other things in Gnus should be altered to support
cleaning it up properly, but it can be fixed up in the meantime.

message and mail-prsvr could be avoided by wrapping references to
`message-posting-charset' and `mail-parse-charset' in
`bound-and-true-p' or by using let forms like

(let ((mail-parse-charset (bound-and-true-p mail-parse-charset)))
   ...)

`mm-body-7-or-8' could be compiled out by making it a defsubst in
mm-bodies and requiring that at compile time.

The quoted-printable functions in mail-utils.el should then be fixed
to use rfc2047.el functions.  Currently `mail-quote-printable' is
wrong:  QP-encoding needs to use the appropriate MIME charset (not
just assume 8859-1) and obey the RFC 2047 rules.  Then the
mail-unquote-... functions only look for a (case sensitive) 8859-1
encoding, which isn't good enough, and presumably they should insert
the appropriate multibyte characters.