netlink: use process threads

[kib]

1. Block all blockable signals on the thread start.
2. You need to handle a situation where the netlink thread goes away while you are trying to schedule a task for it
3. Are netlink socket passable between processes? What should happen if you pass the socket and the receiving process start issuing requests to the other process' socket?

sys/netlink/netlink_domain.c

49

Properly move this to the include block above. But why do you need this header at all?

279

I think these blank lines are not needed.

sys/netlink/netlink_io.c

313

for (;;) {

315

if (error != 0 || thread_check_susp(curthread, true) != 0)
    NLP_LOCK();
    break;
}