Fix a bug that the gateway address was not shown when a -gateway modifier
was used.
Add :ifname modifier to specify interface-specific routes into
{,ipv6_}static_routes and rc.d/routing.  For example:

 static_routes="foo bar:em0"
 route_foo="-net 10.0.0.0/24 -gateway 192.168.2.1"
 route_bar="-net 192.168.1.0/24 -gateway 192.168.0.2"

At boot time, all of the static routes are installed as before.
The differences are:

- "/etc/rc.d/netif start/stop <if>" now configures static routes
  with :<if> if any.
- "/etc/rc.d/routing start/stop <af> <if>" works as well.  <af> cannot be
  omitted when <if> is specified, but a keyword "any" or "all" can be used
  for <af> and <if>.