Small refractory thread code.
Change. `zthreads.context` create new context if you call it from main thread.
Add. `zthreads.xrun` and `zthreads.xfork` which always use global context.

```Lua
local thread, pipe = zthreads.xfork(function(pipe)
  pipe:send("READY")
  pipe:recv()
end)
thread:start()

pipe:recv()
pipe:send("START")
```