Abolish variable `mime-edit-signing-type' and

Abolish variable `mime-edit-signing-type' and
`mime-edit-encrypting-type'.
(mime-edit-process-multipart-1): Separate "signed" to "pgp-signed" and
"kazu-signed"; separate "encrypted" to "pgp-encrypted" and
"kazu-encrypted";
(mime-edit-enclose-signed-region): Renamed from
`mime-edit-enclose-pgp-signed-region'.
(mime-edit-enclose-pgp-encrypted-region): Renamed from
`mime-edit-enclose-encrypted-region'.
(mime-edit-enclose-kazu-signed-region): New function.
(mime-edit-enclose-kazu-encrypted-region): New function.
(mime-edit-set-sign): Don't refer `mime-edit-signing-type'.
(mime-edit-set-encrypt): Don't refer `mime-edit-encrypting-type'.