Support for Lua's I/O library

This can only open files in luafiles/ and its subfolders, and only a few file extensions are allowed.

The primary function for opening a file in read mode is io.open(string filename, string filemode, function callback), which is just like the one from vanilla Lua, except instead of returning a file handle, the host first sends the file to every client, and then call the callback for everyone.

The callback is of the following form: function(file, filename). Inside the callback, you can just use the regular I/O functions like base Lua. The file is automatically closed after the callback returns.

Currently, only files on the server's side can be opened in a synced way, although I might decide to change that in a later patch.

For writing to a file, you can use the io.openlocal(filename, filemode) function, which behaves exactly like io.open() from vanilla Lua (safe for the luafiles/ and whitelist restrictions). Less importantly, io.openlocal() can be used for reading a file locally, although this is only to be used in exceptional circumstances, such as reading a list of passwords without sharing them with the clients, etc.

For extra safety, clients can only open files that are in luafiles/client/ and its subfolders.

...And of course io.popen is gone :V