Fix forking

1. When forking, don't repond to a pause request in the parent after the child has already responded

2. use waitpid() instead of wait() because wait can race with other wait() calls in the program

stub/cbits/stub.cpp

@@ -218,7 +218,7 @@ static StgStablePtr rts_saved_closure = NULL;
 
 
 extern "C"
-void pause_mutator() {
+void pause_mutator(Response &resp) {
   trace("pausing mutator\n");
   pid_t pid;
   if (use_fork){
@@ -233,10 +233,11 @@ void pause_mutator() {
         r_poll_pause_resp->finish(RESP_OKAY);
     }
     paused = true;
+    resp.finish(RESP_OKAY);
   }
   else {
     int status = 0;
-    wait(&status);
+    waitpid(pid,&status,0);
     use_fork = false;
   }
 }
@@ -412,8 +413,7 @@ static int handle_command(Socket& sock, const char *buf, uint32_t cmd_len) {
             resp.finish(RESP_ALREADY_PAUSED);
         } else {
             trace("fork?: %s\n", use_fork ? "yes" : "no");
-            pause_mutator();
-            resp.finish(RESP_OKAY);
+            pause_mutator(resp);
         }
         break;