replace exit(3) by _exit(2) in signal handler sigchld()

exit(3) is not async-signal-safe but, _exit(2) is.
This change prevents st to crash and dump core.

st.c

@@ -730,7 +730,7 @@ sigchld(int a)
 		die("child exited with status %d\n", WEXITSTATUS(stat));
 	else if (WIFSIGNALED(stat))
 		die("child terminated due to signal %d\n", WTERMSIG(stat));
-	exit(0);
+	_exit(0);
 }
 
 void