Add self-encrypt options for PGP and S/MIME.

Add self-encrypt options for PGP and S/MIME.

Add $pgp_self_encrypt, $pgp_self_encrypt_as, $smime_self_encrypt,
$smime_self_encrypt_as.

$pgp_sign_as and $smime_default_key are inappropriate to use, as they
specify signing key/certs.  In some cases, this is a different value
than the encryption key/cert.