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

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.