Use *open_proc* where possible

Using open_proc/fopen_proc/__open_proc is better since
 - it uses openat
 - it comes with nice error reporting

Let's use it in places where we can. Even if it does not give any
improvements (such as in cr-check.c), error message unification
is good enough reason to do so.

Requested-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>