ctrools: Rewrite task/threads stopping engine

Stopping tasks with STOP and proceeding with SEIZE is actually excessive --
the SEIZE if enough. Moreover, just killing a task with STOP is also racy,
since task should be given some time to come to sleep before its proc
can be parsed.

Rewrite all this code to SEIZE task and all its threads from the very beginning.

With this we can distinguish stopped task state and migrate it properly (not
supported now, need to implement).

This thing however has one BIG problem -- after we SEIZE-d a task we should seize
it's threads, but we should do it in a loop -- reading /proc/pid/task and seizing
them again and again, until the contents of this dir stops changing (not done now).

Besides, after we seized a task and all its threads we cannot scan it's children
list once -- task can get reparented to init and any task's child can call clone
with CLONE_PARENT flag thus repopulating the children list of the already seized
task (not done also)

This patch is ugly, yes, but splitting it doesn't help to review it much, sorry :(

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>

include/image.h

@@ -317,9 +317,10 @@ struct core_entry {
 	};
 } __packed;
 
-#define TASK_ALIVE	0x1
-#define TASK_DEAD	0x2
-#define TASK_STOPPED	0x3 /* FIXME - implement */
+#define TASK_SHOULD_BE_DEAD	0x0
+#define TASK_ALIVE		0x1
+#define TASK_DEAD		0x2
+#define TASK_STOPPED		0x3 /* FIXME - implement */
 
 #endif /* CONFIG_X86_64 */