Specifically set sigchld handler to SIG_IGN

See https://pubs.opengroup.org/onlinepubs/9699919799/functions/_Exit.html#tag_16_01_03_01

dwmblocks.c

@@ -108,7 +108,7 @@ setupsignals()
         /* to prevent forked children from becoming zombies */
         sa.sa_flags = SA_NOCLDSTOP | SA_NOCLDWAIT | SA_RESTART;
         // sigemptyset(&sa.sa_mask);
-        sa.sa_handler = SIG_DFL;
+        sa.sa_handler = SIG_IGN;
         sigaction(SIGCHLD, &sa, NULL);
 
         /* to handle signals generated by dwm on click events */