Yet again -- CRIU has an optimized openat()-based engine that
is slightly faster for opening "/proc" files rather than plain
open(). The latter is provided by default by compel.

travis-ci: success for compel: Contrinue improving library
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>