Change zone set/get options related to notify

Change zone set/get options related to notify

Add a type to all dns_zone_(get|set) functions that apply to sending
notifies, so the options can be set and retrieved separately per type.

This affects dns_zone_setnotifydefer, dns_zone_getnotifydefer,
dns_zone_setnotifydelay, dns_zone_getnotifydelay,
dns_zone_setnotifysrc4, and dns_zone_setnotifysrc6.

The functions dns_zone_getnotifysrc4 and dns_zone_getnotifysrc6 are
unused and can be removed.