Rework how fib_priority is handled.

Rework how fib_priority is handled.
Instead of passing it around all the time put the fib_priority into the
kroute state. It is only needed in send_rtmsg() in the end.
Additionally insert F_BGP_INSERTED routes with a special RTP_MINE priority.
This makes changing the fib_priority at runtime simpler because there
is no need to alter the kroute table anymore.
OK tb@ deraadt@