Add. *r pattern to receive available data from socket.

`sock:receive("*r")` could be done with:
``` Lua
function read_some(s)
  local msg, err, part = s:receive("*all")
  if (not msg) and (err ~= 'timeout')  then
    return nil, err
  end
  msg = msg or part
  if not msg then return nil, 'timeout' end
  return msg
end

sock:settimeout(0)
read_some(sock)
```
Merge branch 'moteus-receive-raw' of https://github.com/moteus/luasocket into moteus-receive-raw
Merge branch 'moteus-receive-raw' into moteus