BUG/MINOR: signals/poller: set the poller timeout to 0 when there are signals

BUG/MINOR: signals/poller: set the poller timeout to 0 when there are signals

When receiving a signal before entering the poller, and without any
activity in the process, the poller will be entered with a timeout
calculated without checking the signals.

Since commit 4f59d3 ("MINOR: time: increase the minimum wakeup interval
to 60s") the issue is much more visible because it could be stuck for
60s.

When in mworker mode, if a worker quits and the SIGCHLD signal deliver
at the right time to the master, this one could be stuck for the time of
the timeout.

This should fix issue #1841

Must be backported in every stable version.