chg: let procutil maintain its own pagerpid

Previously, chg.c maintains the pagerpid. Let's move it to procutil.c.

Note: chg.c still have a pagerpid to decide whether to call attachio or not.
In the future, attachio may be moved from hgc_open to hgc_runcommand, and
hgc_runcommand handles both pager and attachio so we don't need to run
attachio twice. And chg.c will be free of pagerpid.

contrib/chg/chg.c

@@ -431,15 +431,14 b' int main(int argc, const char *argv[], c'
 
 	setupsignalhandler(hgc_peerpid(hgc), hgc_peerpgid(hgc));
 	const char *pagercmd = hgc_getpager(hgc, argv + 1, argc - 1);
-	pagerpid = setuppager(pagercmd);
+	pid_t pagerpid = setuppager(pagercmd);
 	if (pagerpid)
 		hgc_attachio(hgc);  /* reattach to pager */
 	int exitcode = hgc_runcommand(hgc, argv + 1, argc - 1);
 	restoresignalhandler();
 	hgc_close(hgc);
 	freecmdserveropts(&opts);
-	if (pagerpid)
-		waitpager(pagerpid);
+	waitpager();
 
 	return exitcode;
 }

contrib/chg/procutil.c

@@ -159,6 +159,7 b' error:'
  * Return 0 if pager is not started, or pid of the pager */
 static pid_t setuppager(const char *pagercmd)
 {
+	assert(pagerpid == 0);
 	if (!pagercmd)
 		return 0;
 
@@ -177,6 +178,7 b' static pid_t setuppager(const char *page'
 				goto error;
 		}
 		close(pipefds[1]);
+		pagerpid = pid;
 		return pid;
 	} else {
 		dup2(pipefds[0], fileno(stdin));
@@ -197,13 +199,16 b' error:'
 	return 0;
 }
 
-static void waitpager(pid_t pid)
+static void waitpager(void)
 {
+	if (pagerpid == 0)
+		return;
+
 	/* close output streams to notify the pager its input ends */
 	fclose(stdout);
 	fclose(stderr);
 	while (1) {
-		pid_t ret = waitpid(pid, NULL, 0);
+		pid_t ret = waitpid(pagerpid, NULL, 0);
 		if (ret == -1 && errno == EINTR)
 			continue;
 		break;