WIP: Multithread some usage of findfile

On a slow disk, descending the directories can be, well, slow. It has also been my suspicion in the past that some timeouts were due to addons loading config files (the exec command uses findfile).

What needs to done:

• Multithread the exec command.
• Create a queue for locating a wad for addfile as well as startup files.
• Create a queue for locating files when connecting to servers.

Although the exec command could be multithreaded in its entirety, wad loading is much more complex. So a filename can be placed in the queue, and another thread handles locating the file. Once the file has been located, its path may be updated in the queue. The the main thread will check the queue every tic and do what it needs to.