uloop: use a waker for notifying sigchld and loop cancel events

Fix a race condition when do_sigchld, uloop_cancelled were set just
before epoll_wait(timeout=-1), resulting the loop stuck in the syscall
without noticing the events just happened

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>