DYNDNS: improve nsupdate_msg_add_fwd()
Update nsupdate_msg_add_fwd() to group commands by address family
processed IP address belongs to.
It's better to group removing old A addresses and adding new A
addresses in a single transaction. Same goes for AAAA addresses.
Separate transaction for A and AAAA addresses updates are important
because server might block updates for one of these families and thus
the update even for the non-blocked address family would unnecessarily
For more details please see:
Reviewed-by: Jakub Hrozek <firstname.lastname@example.org>