update_tls scans sendmail.mc for a line like
include(`/etc/mail/tls/starttls.m4')dnl
and if not found, a warning message is printed and /etc/mail/tls/starttls.m4
is overwritten.

We use only the "submit" portion of sendmail, with client certificate
authentication to a mail hub. Now we have to add the line to sendmail.mc,
even though this file is not used for anything alse.

If update_tls could additionally check submit.mc for then line, and set REFD=1
also in that case, we would not have to make any changes to sendmail.mc at all.

Thanks
Arne