[CRIU] SWRK mode legalization

[Pavel Emelyanov]

Hi,

As you know there are currently 2 ways of using CRIU -- one via CLI and another
one by starting a CRIU service and talking to it using RPC commands (and a .so
which provides C function wrappers around RPC calls).

We have yet another undocumented way to use CRIU which is called SWRK. In this
mode CRIU is fork+exec-ed as it if is about to be used as CLI tool, but instead
of passing arguments via CLI, caller gives to CRIU a socket and sends RPC
commands directly into it. IOW, swrk mode is a mixture of CLI and RPC -- one
doesn't have to start a service, but can request CRIU with easier-to-use RPC
commands.

The mode is not documented as the way to start one seemed not nice from my
perspective, it's like this

   int sk[2];
   socketpair(PF_UNIX, SOCK_SEQPACKET, 0, sk);
   if (fork() == 0) {
         close(sk[0]);
         sprintf(buf, "%d", sk[1]);
         execl("criu", "criu", "swrk", buf, NULL);
   }
   close(sk[1]);
   /* start feeding RPC commands into sk[0] here */

Originally this mode was done to make life for the P.Haul easier, but right now
it looks like integration with Docker would be much easier if done in swrk way.
LXC can probably also benefit from it.

So I think it's a good time to stop hiding it, nail down the API and use in
Docker (and probably LXC).

Having nothing better as the above code snippet in mind, I thought to just add
a couple of argv[] sanity checks to criu and declare this as official swrk API.
If you have suggestions how to make it better -- you're warmly welcome :)

The RPC is described here (http://criu.org/RPC) and there's a small section
about swrk in the bottom of the page.

-- Pavel