Add support for PC-card router configuration. (MFPAO)

o Add the removable_interfaces variable for list of removable network

interfaces (PC-card ethernet, wireless network and USB ethernet etc).

o ifconfig_<ifn>_alias0, static_routes_<ifn>, removable_route_flush,

/etc/start_if.<ifn> and /etc/stop_if.<ifn> are support.

o removable_route_flush variable is set to "NO" if you want to use the

machine as gateway using two or more removable network cards. If
static routing is needed use static_routes_<ifn> instead of
static_routes or defaultrouter.

o The optional static_routes_<ifn> variable is likely static_routes.
o /etc/start_if.<ifn> and /etc/stop_if.<ifn> are shell script to be

specified that are called when a card is inserted or removed.