service: add ability to set inherit file descriptors (v3)

This is required to use criu swrk in libcontainer.

v2: remove useless function declaration
    allow to set inherit_fd only for swrk
v3: check swrk out of loop

Cc: Saied Kazemi <saied@google.com>
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Andrew Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>