iproute2: module to create rt_table file & co

When doing source routing/multihoming, it's practical to give names to routing
tables. The absence of the rt_table file in /etc make this impossible.
This patch recreates these files on rebuild so that they can be modified
by the user see NixOS#38638.

iproute2 is modified to look into config.networking.iproute2.confDir instead of
/etc/iproute2.