emacs: fix MML quoting in replies

The reply MML quoting added in commit ae438cc unintentionally MML
quotes also the signature/encryption MML tags added via
message-setup-hook, causing the reply not to be signed/encrypted.

MML quote just the original message in the temp buffer before
inserting it to the message buffer, to not interfere with message mode
hooks or message construction in general.

See [1] and [2] for bug reports.

Thanks to Tim Bielawa <tbielawa@redhat.com> for testing.

[1] id:"87hay78x6l.fsf@wyzanski.jamesvasile.com"
[2] id:"1330812262-28272-1-git-send-email-tbielawa@redhat.com".

Signed-off-by: Jani Nikula <jani@nikula.org>