This commit changes the behavior for the wan-dnsmasq to read the
interfaces to update the DNS servers on from flag-files.

This way, external custom packages which add a custom VPN WAN interface
can update the DNS server gluon is using for WAN resolving.

[Matthias Schiffer: fix up start/stop order and file permissions]

Using service_start requires exporting of environment variables.

Fixes #397

This avoids poking dnsmasq every time a DHCP lease is obtained or a router
advertisement received.

This package provides a secondary dnsmasq instance on port 54 which resolves
everything over the WAN interface.