daemon: graceful shutdown warning and limit removal
git clones may take longer than 30s, much longer... So prepare
to wait almost indefinitely for sockets to timeout and document
the second signal behavior for immediate shutdown.
While we're at it, move parent death handling to a separate
class to avoid Danga::Socket->AddOtherFds, since that does not
allow proper handling the parent pipe being closed and would
actually misterminate a worker prematurely. t/nntpd.t is update
to illustrate the failure with workers enabled.
We will work to keep memory usage low and let clients take their
time without interrupting them.