Change $postpone_encrypt to use self-encrypt variables first.

Change $postpone_encrypt to use self-encrypt variables first.

The concept of postpone encryption is similar to self-encrypting on
send.  The problem is the $postpone_encrypt_as option doesn't take
into account whether PGP or S/MIME is being used.

Since we need to add the new options for self-encryption, modify
$postpone_encrypt to use them by default instead, falling back to
$postpone_encrypt_as if they are unset.  Note $postpone_encrypt_as is
deprecated in the documentation.