don't spoil errno in sig handler (#411)

reported by thread-sanitizer.

the sighandler's spoiled `errno` was causing xlib to incorrectly assume some
error occurred and thus causing the crash described in #391.

to reproduce:

* Open an nsxiv window
* Open another terminal and run the following:

	var=$(pidof nsxiv); while :; do kill -s SIGCHLD $var; done

putting the `pid` into a variable is actually important because doing
`$(pidof nsxiv)` inside the loop makes it really hard to reproduce the
issue, I presume because of the extra process invocation it was sending
less SIGCHLD and so putting it into a variable avoids that overhead and
is able to generate more signals.

instead of reaping the zombies manually, we now pass the
`SA_NOCLDSTOP|SA_NOCLDWAIT` for SIGCHLD instead so that the zombies are
reaped automatically.

Closes: https://codeberg.org/nsxiv/nsxiv/issues/391
Reviewed-on: https://codeberg.org/nsxiv/nsxiv/pulls/411
Reviewed-by: explosion-mental <explosion-mental@noreply.codeberg.org>

main.c

@@ -843,19 +843,14 @@ static void run(void)
 	}
 }
 
-static void sigchld(int sig)
+static void setup_signal(int sig, void (*handler)(int sig), int flags)
-{
-	while (waitpid(-1, NULL, WNOHANG) > 0);
-}
-
-static void setup_signal(int sig, void (*handler)(int sig))
 {
 	struct sigaction sa;
 
 	sa.sa_handler = handler;
 	sigemptyset(&sa.sa_mask);
-	sa.sa_flags = SA_RESTART | SA_NOCLDSTOP;
+	sa.sa_flags = flags;
-	if (sigaction(sig, &sa, 0) == -1)
+	if (sigaction(sig, &sa, NULL) < 0)
 		error(EXIT_FAILURE, errno, "signal %d", sig);
 }
 
@@ -865,8 +860,8 @@ int main(int argc, char *argv[])
 	size_t n;
 	const char *homedir, *dsuffix = "";
 
-	setup_signal(SIGCHLD, sigchld);
+	setup_signal(SIGCHLD, SIG_DFL, SA_RESTART|SA_NOCLDSTOP|SA_NOCLDWAIT);
-	setup_signal(SIGPIPE, SIG_IGN);
+	setup_signal(SIGPIPE, SIG_IGN, 0);
 
 	setlocale(LC_COLLATE, "");