Introduce 'pr_usrreqs' structure and move existing user-protocol

Introduce 'pr_usrreqs' structure and move existing user-protocol
handlers into it. We want to split existing (*pr_usrreq)() to multiple
short handlers for each PRU_ request as it was already done for
PRU_ATTACH and PRU_DETACH. This is the preparation step, (*pr_usrreq)()
split will be done with the following diffs.

Based on reverted diff from guenther@.

ok bluhm@