crtools: Add internal "swrk" action

To help restoring tasks from images as kids to the caller, we can
do the trick.

1. Caller sets himself as child reaper with PR_SET_CHILD_SUBREAPER prctl
2. Caller makes sure criu binary is suid-ed and owned by root
3. Caller forks and calls execv() on criu asking it to restore
4. Criu finishes restore and exits. All its kids get reparented to the
   criu's parent, i.e. -- to the library caller.
5. Caller stops being subreaper

In order to make the execv() and arguments passing simpler I propose
to execv() the service worker function, that accepts options via socket.

This is good for two reasons.

1. We don't have to construct CLI options in libcriu
2. We reuse other service's facilities, such as security checks,
   ability to dump, pre-dump and other stuff

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