service: Add ability to inherit page server socket

The swrk action is turning out to be a cool thing. We can
spawn criu with swrk action with some FD being open, then
ask for dump/pre-dump/page-server telling it that some
descriptor it needs is "out there".

This patch lets us specify that the page server communication
channel is already in criu's fdtable.

TODO: teach regular service to accept fd via service socket.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>