Rename pgpring to mutt_pgpring.

Rename pgpring to mutt_pgpring.

pgpring is used by another common package, signing-party.  At the
request of ArchLinux, I am renaming our version to be prefixed by
"mutt_" in order to remove a naming conflict.

This is a minor inconvenience for pgp2, pgp5, and pgp6 users, but I
believe this constitutes an extremely small number of users at this
point.

I am keeping pgpewrap as-is because it is also used by GnuPG, and has
no such naming conflict.