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>