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.