rtsock: do not use raw socket code

This makes routing socket implementation self contained and removes one
of the last dependencies on the raw socket code and pr_output method.

There are very subtle API visible changes:

• now routing socket would return EOPNOTSUPP instead of EINVAL on syscalls that are not supposed to be called on a routing socket.
• routing socket buffer sizes are now controlled by net.rtsock sysctls instead of net.raw. The latter were not documented anywhere, and even Internet search doesn't find any references or discussions related to these sysctls.

Reviewed by: melifaro
Differential revision: https://reviews.freebsd.org/D36122