Only write v4 NAT rules for networks that actually have a v4 address.
Write new dnsmasq configuration on every config push.

This is mainly to support network hotplugging.  If a network is deleted, the
current driver implementation leaves the old dnsmasq configuration for that
interface laying around (and causes dnsmasq to fail to start, because the
associated interface is gone).
Merge pull request #82 from ryanpetrello/linux

Fix a few issues that can unintentionally cause the router to reboot as networks are modified.