Speed up service-fd retrieval

We're using get_service_fd in file engine,
better to make it fast. This patch caches
the limits system provides us, instead of
calling getrlimit() every time.

This patch introduces is_service_fd helper
which will be used instead of get_service_fd
where it make sense.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>