Put signatures at the very end of the message

Put signatures at the very end of the message

The existing code inserts the signature before inserting the message
body (which it puts at the very end of the buffer - therefore AFTER
the signature). This little snippet makes us search backwards and
insert the message body before a signature, if it exists.

This also fixes a small indentation issue in David's code.

Signed-off-by: Dirk Hohndel <hohndel@infradead.org>