global/signal_handler: use poll(2) instead of select(2)

Starting with commit 61a298c39c1a6684682e2b749e45a66d073182c8 we delay the
signal handler setup until after lots of other initialization has happened,
which can result in us having very large (>1024) open fds, which will
break the FD_SET macros for select(2).  Use poll(2) instead.

Fixes: #5722
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Yehuda Sadeh <yehuda@inktank.com>