ps: ignore SIGURG

Stop registering signal handler for SIGURG, to avoid ps failure if
someone sends such signal. Without the signal handler, SIGURG will
just be ignored.

  Signal 23 (URG) caught by ps (3.3.16).
  ps:ps/display.c:66: please report this bug

https://man7.org/linux/man-pages/man7/signal.7.html
https://www.freebsd.org/cgi/man.cgi?sektion=3&query=signal

ps/display.c

@@ -638,6 +638,7 @@ int main(int argc, char *argv[]){
     case SIGKILL:  /* can not catch */
     case SIGSTOP:  /* can not catch */
     case SIGWINCH: /* don't care if window size changes */
+    case SIGURG:   /* Urgent condition on socket (4.2BSD) */
       ;
     }
   } while (0);