Added options to configure multiple instances of dnsmasq.

The easiest (and sometimes only) way to allow for more complex dnsmasq configurations is to run multiple instances. I needed some overrides for one interface, but not for the others. Moreover, having the same hostname on multiple networks is quite troublesome with only one instance. The solution was to filter the hosts file based on the interfaces dnsmasq is configured to listen to.

Note: A more elegant solution to filter the hosts file is to modify dhcpleases. But that requires changing a binary, which is in the tools repository.