Correctly recursively parse RFC-822 formatted MIMEs

It turns out that some clients (Krste... :() just bail on the whole
MIME thing and jam a preformatted message into a mail in order to
forward things.  This solves the problem by recursively parsing these
sorts of messages.
Buff up the MIME plain-text body finder

This now handles the very complicated message that Krste sent me
earlier this week.  It still doesn't bother to decode
quoted-printable, but I guess I'll have to do that next so I can
reply... :)
Remove an accidental debugging printf