A service fd should be created, otherwise get_service_fd returns -1.

This patch removes this functionality from other subsystems and
allows to clone service descriptors.

v2: rename open_service_fd to install_service_fd
v3: two patches were merged for bisecting
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>