Currently, the documentation states that, with the exception of
pgp_getkeys_command, all the PGP format commands should be available to
the pgp_*_command configuration variables.

However, the %a format is actually only enabled for
pgp_clearsign_command, pgp_encrypt_sign_command, and pgp_sign_command.

Philip Rinn discovered this problem when he was trying to add a
"--encrypt-to %a" to pgp_encrypt_only_command.  Rather than just fix
the one configuration he was having the problem with, this patch
enables %a usage for all of them. (With the exception of the documented
pgp_getkeys_command.)