[Networking] More network script changes

Only kill the necessary dhclient processes rather than all of them.

Kill ifplugd for eth0 and wlan0.  Set "auto eth0" and "allow-hotplug
wlan0" since there is no ifplugd for these interfaces.

Use a virtual FPP-wlan0 interface for wlan0 so we can support using
a static IP on wlan0.