shutdown: Simplify shutdown_signal

shutdown: Simplify shutdown_signal

There is no signal 0 (kill(pid, 0) has special semantics to probe whether
a process is alive), rather than actually sending a signal 0).  So we
can use the simpler 0, instead of -1, for our sentinel of whether a
shutdown request due to a signal has happened.

Suggested-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Message-Id: <20170515214114.15442-2-eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>