Use SIG_IGN for SIGCHLD instead of our own handler

The behavior of handlers registered with signal(3p) is not well-defined
for signals delivered more than once, as laid out in the man page.

We should replace our use of signal with sigaction, but for SIGCHLD
specifically we can also just skip the signals altogether by setting the
handler to SIG_IGN which causes child reaping to not be required.

Fixes: https://github.com/swaywm/sway/pull/8567

include/sway/server.h

@@ -163,6 +163,7 @@ bool server_start(struct sway_server *server);
 void server_run(struct sway_server *server);
 
 void restore_nofile_limit(void);
+void restore_signals(void);
 
 void handle_new_output(struct wl_listener *listener, void *data);