libcontainer: sync: cleanup synchronisation code

This includes quite a few cleanups and improvements to the way we do
synchronisation. The core behaviour is unchanged, but switching to
embedding json.RawMessage into the synchronisation structure will allow
us to do more complicated synchronisation operations in future patches.

The file descriptor passing through the synchronisation system feature
will be used as part of the idmapped-mount and bind-mount-source
features when switching that code to use the new mount API outside of
nsexec.c.

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

tty.go

@@ -100,7 +100,7 @@ func (t *tty) initHostConsole() error {
 }
 
 func (t *tty) recvtty(socket *os.File) (Err error) {
-	f, err := utils.RecvFd(socket)
+	f, err := utils.RecvFile(socket)
 	if err != nil {
 		return err
 	}