emacs: fix notmuch-mua-reply point placement when signature involved

emacs: fix notmuch-mua-reply point placement when signature involved

When composing a reply, notmuch-mua-reply attempts to  cite the
the original message by inserting it before the user signature, if
one is present. The existing method used to search the signature
separator backward from the end of the buffer and then move one
line up. In case of variable `message-signature-insert-empty-line'
being nil this caused point to go to the beginning of
'--text follows this line--'
separator line, and citation was inserted there.
This change checks the value of `message-signature-insert-empty-line'
and doesn't move point if that is nil. Additional narrowing to
the body region ensures that point never goes to the separator line
(or beyond).
`message-signature-setup-hook' or `message-setup-hook' may already have
added some other content to the message body, therefore using simply
(message-goto-body) to move point to the beginning of body might lead
to unexpected results.

Original patch from "Geoffrey H. Ferrari", continued with iterations
from Jani and Mark.