runc: remove --criu option

This was introduced in an initial commit, back in the day when criu was
a highly experimental thing. Today it's not; most users who need it have
it packaged by their distro vendor.

The usual way to run a binary is to look it up in directories listed in
$PATH. This is flexible enough and allows for multiple scenarios (custom
binaries, extra binaries, etc.). This is the way criu should be run.

Make --criu a hidden option (thus removing it from help). Remove the
option from man pages, integration tests, etc. Remove all traces of
CriuPath from data structures.

Add a warning that --criu is ignored and will be removed.

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

utils_linux.go

@@ -47,7 +47,6 @@ func loadFactory(context *cli.Context) (libcontainer.Factory, error) {
 	}
 
 	return libcontainer.New(abs, intelRdtManager,
-		libcontainer.CriuPath(context.GlobalString("criu")),
 		libcontainer.NewuidmapPath(newuidmap),
 		libcontainer.NewgidmapPath(newgidmap))
 }