minimize pause during graceful restart for server.max-worker = 0 case
The previous generation continues to accept new connections until the
restarted parent signals that the restarted server is ready to accept
new connections, and so the previous server should gracefully shutdown.
This does not apply in the case of multiple workers.
When there are multiple workers, they receive SIGINT to gracefully shut
down and stop accepting new connections. While the listen sockets are
kept open (and not closed and reopened), there is a small pause while
the parent process restarts before it begins accepting new connections
from the listen backlog.
Note: there is a window during restart during which lighttpd may exit
if it receives certain signals before it sets up signal handlers.
future: might block signals (sigprocmask()) during restart, but if that
is done, then care must be taken to unblock signals in restarted server
as soon as signal handlers are set up and before any other children are
created, e.g. by modules, or else signals must be explicitly unblocked
in children. Also, during command line and config file processing,
signals would be blocked, too, which might not be ideal.