ifupdown: support metric for static default gw

ifupdown: support metric for static default gw

This is useful when you have multiple ISPs with failover. It allows
setting the priority of the static gateway and makes it possible
to specify multiple static gateways.

The ubuntu ifupdown supports it.

function                                             old     new   delta
.rodata                                           116725  116797     +72
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 1/0 up/down: 72/0)               Total: 72 bytes
   text    data     bss     dec     hex filename
 953343    7313    8984  969640   ecba8 busybox_old
 953415    7313    8984  969712   ecbf0 busybox_unstripped

Signed-off-by: Natanael Copa <natanael.copa@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>