rtsock: do not use raw socket code

[glebius]

Summary

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.

[melifaro]

sys/net/rtsock.c

295–300

Nit: worth having a separate function to simplify code here / in the last condition.

[glebius]

sys/net/rtsock.c

295–300

I'd rather remove the optimization with not doing m_copym() for the last mbuf. This isn't high performance path and IMHO the added complexity isn't justified. But in a separate change.

[melifaro]

sys/net/rtsock.c

295–300

It's often just one active (routing daemon) rtsock socket in the system receiving the events.

[glebius]

sys/net/rtsock.c

295–300

Will do!