compel: Add more arguments to compel_wait_task()

Some get_status() methods may allocate data, because
not all of the fields in /proc/[pid]/status file
have the fixed size. For example, NSpid, which
size may vary.

Introduce new method free_status() in counterweight
for such type get_status() methods. it will be called
in case of we go to try_again and need to free allocated
data.

Also, introduce data parameter for a use in the future.

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>