Fixes to require, loadlib and UdpServer

[Dregu]

It was recently brought back to my attention that loading DLLs with require simply didn't work at all, and loadlib could only load them from the game root.

• Fixed loading DLLs with require from the mod folder
• Fixed loading DLLs with package.loadlib from the mod folder
• Fixed silly 3rd party scripts using require on built-in libraries just setting them to nil
• Added UdpServer:close() (and garbage collection by nil) to close servers
• Added UdpServer:open() and UdpServer:error() to check what went wrong

This should allow things like the archipelago mod to simply require "apclientpp" when apclientpp.dll is vendored with the mod, instead of all the extra steps when installing it.

The following also seem to work for example, when a complete luasocket build is provided to the mod dir:

-- Loads socket.lua, which later requires the actual core.dll with require("socket.core"), which all seems to work out
local socket = require("socket")
local mime = package.loadlib("mime/core.dll", "luaopen_mime_core")()

[Mr-Auto]

shouldn't this be thread_local ?

[Mr-Auto]

Have you though about making safe version of udp_listen? so people don't have to worry about it being on different thread

[Mr-Auto]

Maybe instead of callback, having a function that you can use to request the current buffer of messages

[Dregu]

I have actually made exactly that locally already, but haven't pushed it cause there's also a bunch of tcp stuff that doesn't work at all.