cgroup: Refactor special_props comparisons

Let's add a function which will tell if cgroup property
is special.

Note that I do not touch restore_special_props() indention
in this patch as I separate subfunction from it in the
next patch, so touching this pile of code once.

Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>