tty: restore sid on master points only (v2)

If a master point has sid, it doesn't mean, that all slave points
will have sid. Look at tty03 for example:

  PID   SID TT       COMMAND
26748 26748 ?        tty03
26749 26749 pts/2     \_ tty03
26750 26750 ?             \_ tty03

The second process has not a file descriptor for the ctl tty,
but this tty is opened in tty03.

v2: If a slave point with sid has not a master point, the option
--shell-job must be set and sid isn't restored for such terminals.
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

test/zdtm.sh

@@ -73,6 +73,7 @@ static/pty00
 static/pty01
 static/pty04
 static/tty02
+static/tty03
 static/child_opened_proc
 static/cow01
 static/fpu00

tty.c

@@ -731,9 +731,11 @@ static int tty_find_restoring_task(struct tty_info *info)
 	}
 
 	if (info->tie->sid) {
-		if (pty_is_master(info)) {
+		if (!pty_is_master(info)) {
 			if (tty_has_active_pair(info))
 				return 0;
+			else
+				goto shell_job;
 		}
 
 		/*
@@ -750,8 +752,7 @@ static int tty_find_restoring_task(struct tty_info *info)
 					       info->tfe->id);
 		}
 
-		if (pty_is_master(info))
-			goto notask;
+		goto notask;
 	} else {
 		if (pty_is_master(info))
 			return 0;
@@ -759,6 +760,7 @@ static int tty_find_restoring_task(struct tty_info *info)
 			return 0;
 	}
 
+shell_job:
 	if (opts.shell_job) {
 		pr_info("Inherit terminal for id %x\n", info->tfe->id);
 		info->tie->sid = info->tie->pgrp = INHERIT_SID;