Change pr_output's signature to take two fixed arguments and possible

Change pr_output's signature to take two fixed arguments and possible
additional varidic arguments. Change the various protocols accordingly.

This is not the signature NetBSD uses (only struct mbuf * + variadic),
but since all protocols use the socket argument (beside NS), it is more
reasonable to keep that static as well.

While at it, fix the pr_soport in ip6protosw to match the protosw.