Support PGP messages longer than 1024 characters

It turns out I've been signing all my messages longer than 1024
characters incorrectly!
Print an error message when PGP doesn't sign a message

This wasn't actually happening (the previous commit fixed the bug),
but I figure this is a decent idea anyway.
Drop an extra ";" from the PGP header

This appears to not actually cause any problems for anyone, but it is
certainly odd.
Strip trailing whitespace from "mhng-comp" messages

This is a nightmare to support for PGP -- I can't quite figure out how
to get other clients to work, and the RFCs are kind of conflicting.  I
think the best thing to do is to just strip trailing whitespace
entirely.