Manage dnsmasq using upstart

This is a fairly substantial change. It:

 * disables the init.d script using /etc/default/dnsmasq
 * adds an upstart script to manage the dnsmasq process
 * always ignores resolvconf configuration (and hence removes the `ignore_resolvconf` option)
 * configures dnsmasq's upstream nameservers directly in `/etc/resolv.conf.dnsmasq` rather than (ab)using `/etc/resolv.conf`

if you want to use resolvconf configuration you can just add `resolv-conf=/var/run/dnsmasq/resolv.conf` to a dnsmasq conf snippet.