dnsmasq: add reload trigger when getting DNS servers from interface
This is an IOWRT addition introduced with: 8f243e7b
It got broken when OpenWRT introduced specific interface triggers: 4bc4c2ea
The regression is addressed by adding explicit reload triggers when DNS servers are retrieved from interfaces.
In addition the logic for determining when to retrieve DNS servers from an interface is reworked so that instead of matching against existing interfaces any valid UCI section identifier is treated a potential interface name. This avoids creating a faulty configuration and a resulting crash loop if for example an interface has not been brought up yet. This should not cause any problems since neither IPv4 nor IPv6 addresses would serve as valid UCI section identifiers.
Solves this ticket: https://project.iopsys.eu/issues/14245