upas/runq: wait for the child process to send back the message if it was rejected

When a message cannot be delivered (eg. when the remote server reply with
a permanent error), the message is sent back to the sender by spawning
a child process (marshal). But runq doesn't wait for this process (returnmail())
and removes the message from disk (see remmatch()).

The original code[1] used wait(), which has been removed from 9front
with the commit ('runq: clean up code, fix error handling.')[2]
which was a fix to the commit ('upas/runq: support parallel queue processing, drop -a')[3].

This commit will change back to wait for marshal
if runq runs with one job (-n 1, which is the default setting)
in order to avoid interfering with jobs created from rundir()[4],
even if I don't understand how.

[1]: http://9p.io/sources/plan9/sys/src/cmd/upas/q/runq.c
[2]: http://git.9front.org/plan9front/plan9front/49d7ca8d92e5667f3e5ece4c6acbc1064701e2f8/commit.html
[3]: http://git.9front.org/plan9front/plan9front/f321298c551e4333fcf2819eabf7ce67ea443e20/commit.html
[4]: https://inbox.vuxu.org/9front/ZB+YbtCfhrrn6WyW@alice/
---
diff 437db15d657ab83cad4fbbe1f146331b0b1b894c 3a1cfac4391588869011879eaca26f0deccbbe04
--- a/sys/src/cmd/upas/q/runq.c
+++ b/sys/src/cmd/upas/q/runq.c
@@ -575,7 +575,8 @@
 returnmail(char **av, Wdir *w, char *name, char *msg)
 {
 	char buf[256], attachment[Pathlen], *sender;
-	int fd, pfd[2];
+	int fd, pfd[2], rc;
+	Waitmsg *wm;
 	long n;
 	String *s;
 
@@ -597,7 +598,7 @@
 		return -1;
 	}
 
-	switch(rfork(RFFDG|RFPROC|RFENVG|RFNOWAIT)){
+	switch(rfork(RFFDG|RFPROC|RFENVG|(njob != 1 ? RFNOWAIT : 0))){
 	case -1:
 		logit("runq - fork failed", w, name, av);
 		return -1;
@@ -625,6 +626,7 @@
 			fprint(pfd[1], "%s ", av[n]);
 	}
 	fprint(pfd[1], "'' failed (code %s).\nThe symptom was:\n\n", msg);
+	rc = 0;
 	fd = open(file(name, 'E'), OREAD);
 	if(fd >= 0){
 		for(;;){
@@ -633,13 +635,28 @@
 				break;
 			if(write(pfd[1], buf, n) != n){
 				close(fd);
-				return -1;
+				rc = -1;
+				goto out;
 			}
 		}
 		close(fd);
 	}
 	close(pfd[1]);
-	return 0;
+out:
+	if(njob == 1){
+		rc = -1;
+		wm = wait();
+		if(wm == nil)
+			syslog(0, runqlog, "wait failed: %r");
+		else{
+			if(wm->msg[0])
+				syslog(0, runqlog, "returnmail child: %s", wm->msg);
+			else
+				rc = 0;
+			free(wm);
+		}
+	}
+	return rc;
 }
 
 /*