Add support for adding default routes for other FIBs

Make rc.d/routing read defaultrouter_fibN and ipv6_defaultrouter_fibN, and
set it as the default gateway for FIB N, where N is from 1 to (net.fibs - 1)
This allows adding gateways for multiple FIBs in the same format as the main
gateway. (FIB 0)

Reviewed by: olivier, rgrimes, bcr (man page)
Sponsored by: ScaleEngine Inc.
Differential Revision: https://reviews.freebsd.org/D22706