runc checkpoint: fix --status-fd to accept fd

1. The command `runc checkpoint --lazy-server --status-fd $FD` actually
accepts a file name as an $FD. Make it accept a file descriptor,
like its name implies and the documentation states.

In addition, since runc itself does not use the result of CRIU status
fd, remove the code which relays it, and pass the FD directly to CRIU.

Note 1: runc should close this file descriptor itself after passing it
to criu, otherwise whoever waits on it might wait forever.

Note 2: due to the way criu swrk consumes the fd (it reopens
/proc/$SENDER_PID/fd/$FD), runc can't close it as soon as criu swrk has
started. There is no good way to know when criu swrk has reopened the
fd, so we assume that as soon as we have received something back, the
fd is already reopened.

2. Since the meaning of --status-fd has changed, the test case using
it needs to be fixed as well.

Modify the lazy migration test to remove "sleep 2", actually waiting
for the the lazy page server to be ready.

While at it,

 - remove the double fork (using shell's background process is
   sufficient here);

 - check the exit code for "runc checkpoint" and "criu lazy-pages";

 - remove the check for no errors in dump.log after restore, as we
   are already checking its exit code.

[v2: properly close status fd after spawning criu]
[v3: move close status fd to after the first read]

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/criu_opts_linux.go

@@ -36,5 +36,5 @@ type CriuOpts struct {
 	EmptyNs                 uint32             // don't c/r properties for namespace from this mask
 	AutoDedup               bool               // auto deduplication for incremental dumps
 	LazyPages               bool               // restore memory pages lazily using userfaultfd
-	StatusFd                string             // fd for feedback when lazy server is ready
+	StatusFd                int                // fd for feedback when lazy server is ready
 }