rc: fix Xpipefd unbalancing the redir stack

Xpipefd wants the pipe descriptor to be closed in turfredir(), so
it pushes the redirection, but this breaks Xpopredir after normal
redirection. so we shuffle the Xpipefd redir to the bottom of the
stack.

sys/src/cmd/rc/exec.c

@@ -123,6 +123,21 @@ pushredir(int type, int from, int to)
 	runq->redir = rp;
 }
 
+void
+shuffleredir(void)
+{
+	redir **rr, *rp;
+
+	rp = runq->redir;
+	if(rp==0)
+		return;
+	runq->redir = rp->next;
+	rp->next = runq->startredir;
+	for(rr = &runq->redir; *rr != rp->next; rr = &((*rr)->next))
+		;
+	*rr = rp;
+}
+
 var*
 newvar(char *name, var *next)
 {

sys/src/cmd/rc/fns.h

@@ -55,6 +55,7 @@ void	readhere(void);
 word*	searchpath(char*);
 void	setstatus(char*);
 void	setvar(char*, word*);
+void	shuffleredir(void);
 void	skipnl(void);
 void	start(code*, int, var*);
 int	truestatus(void);

sys/src/cmd/rc/havefork.c

@@ -185,7 +185,8 @@ Xpipefd(void)
 	default:
 		addwaitpid(pid);
 		close(sidefd);
-		pushredir(ROPEN, mainfd, mainfd);	/* isn't this a noop? */
+		pushredir(ROPEN, mainfd, mainfd);
+		shuffleredir();	/* shuffle redir to bottom of stack for turfredir() */
 		strcpy(name, Fdprefix);
 		inttoascii(name+strlen(name), mainfd);
 		pushword(name);