global/signal_handler: poll on the control pipe, too

We also need to poll the control fd/pipe so that we restart the poll loop
when new signal handlers are added.  This was broken by commit 8e4a78f1.

Fixes: #5742
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Yehuda Sadeh <yehuda@inktank.com>