keysock: do not use raw socket code

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

Reviewed by: melifaro
Differential revision: https://reviews.freebsd.org/D36123