syscalls from loadable modules compiled in statically into the kernel

[Mateusz Guzik]

The kernel has the following mechanism:

int             
syscall_thread_enter(struct thread *td, struct sysent *se)
{               
        u_int32_t cnt, oldcnt;
                
        do {            
                oldcnt = se->sy_thrcnt;
                if ((oldcnt & SY_THR_STATIC) != 0)
                        return (0);
                if ((oldcnt & (SY_THR_DRAINING | SY_THR_ABSENT)) != 0)
                        return (ENOSYS);
                cnt = oldcnt + SY_THR_INCR;
        } while (atomic_cmpset_acq_32(&se->sy_thrcnt, oldcnt, cnt) == 0);
        return (0);
}

Except it turns out that it is used even if given module (here: sysvshm) is
compiled in statically.

So my proposal is to give modules an easy way to tell whether they got
compiled in and extend syscall_register interface so that it would allow
registering static syscalls.

The latter could also be used by modules which are loadable, but don't
support unloads.

I don't have any good idea how to provide aforementioned detection
method though.

Also, please see https://reviews.freebsd.org/D1007 which moves
SY_THR_STATIC check to an inline function, saving us 2 function calls on
each syscall.

-- 
Mateusz Guzik <mjguzik gmail.com>