Introduce the pru_*() wrappers for corresponding (*pr_usrreq)() calls.

Introduce the pru_*() wrappers for corresponding (*pr_usrreq)() calls.

This is helpful for the following (*pr_usrreq)() split to multiple
handlers. But right now this makes code more readable.

Also add '#ifndef _SYS_SOCKETVAR_H_' to sys/socketvar.h. This prevents the
collisions when both sys/protosw.h and sys/socketvar.h are included
together. Both 'socket' and 'protosw' structures are required to be
defined before pru_*() wrappers, so we need to include sys/socketvar.h to
sys/protosw.h.

ok bluhm@