pru & stat(2) failure / return cleanups

[Tyler Retzlaff <rtr%netbsd.org@localhost>]

hello,

recently effort is being undertaken to separate out functions being
dispatched through the generic pr_usrreq() switches. as a result some
inconsistencies have been identified in failure / return of PRU_SENSE
requests (i.e. stat(2)).

issue #1 so_pcb is NULL

    across the range of protocols there is inconsistency in whether or not
    the so_pcb being NULL is an error.

    for protocols that allocate a pcb in attach should it ever be valid
    for socket->so_pcb to be NULL (except upon entry to attach)?

    currently only tcp & rip6 deviate from this expectation. tcp because of
    pr/46077 (which made PRU_SENSE not fail) and rip6 is probably just wrong.


issue #2 PRU_SENSE / stat(2) returning success, a lot

    most implementations of pr_stat don't fill any values / change the
    passed in struct stat *ub in any way but some of them return 0
    (success) and some of them return EOPNOTSUPP.

    i'd like to suggest that for all the cases where struct stat *ub is
    not being filled in they be changed to EOPNOTSUPP because it seems
    a bit wrong to do nothing at all and then claim success.


comments, clarification, education invited

thanks