Use openat() helper intensively when opening /proc/pid/X files

This allows us to get rid of open-coded "/proc/pid/X".

Based-on-patch-from: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: Pavel Emelyanov <xemul@parallels.com>