support freebsd's fib feature

[cnbatch]

This patch is to support FreeBSD's FIB feature.

When using FreeBSD's setfib command to launch OpenVPN in other FIB (routing table), the interface's IP address was not added to correct routing table, and then the routing entries couldn't be added successfully: https://forums.freebsd.org/threads/freebsd-14-and-route-in-non-zero-fib.91099/ https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=278295

This is duo to ifconfig command does not retrieve FIB number of current envirnment which is set by setfib. It only accepts fib N parameter. route command can retrieve FIB number of current envirnment, the following routing entries will be added to correct routing table. But if the interface's IP address was not added correct routing table, route command will fail.