Api function dns_rr_add updates serial before insert

In remote.d/dns.inc.php the code sequence is not optimal.

When a $update_serial is passed as true, it will now first update the serial (which triggers an update on a slave server) ... and then insert the newly added record. Which then triggers another zone update on the slave, without a serial increase.

Similar to other functions such as dns_rr_update() this should be the other way around.