ssl-priorities not obeyed

It looks like tls.ssl-priorities is not obeyed, at least in imap4d.  I
think copy_conf needs to copy this in addition to the certificate,
key, and ca file names.

Also, if this assumption is not supplied, I think it would be better to
use the gnutls default priorities instead of explicitly passing NORMAL,
as it does now.  Then if the system has changed the defaults, e.g., in
/etc/gnutls/config, they would take effect.

It would also be nice for the documentation to give a link to the format
of this option, for example to
https://gnutls.org/manual/html_node/Priority-Strings.html

Thanks.

                                        Ken