s3:utils: Use lp_dns_hostname() for 'net' dns updates

s3:utils: Use lp_dns_hostname() for 'net' dns updates

name_to_fqdn() requires /etc/hosts to be set up in a special way to find
out the fqdn for dns updates. They are not set up by default and the
DNS update fails. Normally the fqdn is just <lp_netbios_name>.<realm>
and we should just use that. However if it is different, you can set
it to the special value in the smb.conf now.

Signed-off-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>