Mounting "*_downloads" directory for every gamedir downloaded resources

[SNMetamorph]

GoldSrc works exactly like this, and it's truly better way to store custom content downloaded from servers, instead of storing it inside "downloaded" folder in game directory.

[a1batross]

I don't like how the root directory becomes messy with all these optional directories. That's why the downloaded/ is inside game directory.

I had plans on supporting mounting _hd and _lv folders, but only because this is an existing convention, that modders and players rely on.

[a1batross]

Also, I just realized it might not be possible to have a download directory outside of game directory because game directory is considered to be a write directory and all write operations happen relative to it.

And we can't have relative access because of FS_CheckNastyPath. It's possible to temporarily lift down this limitation with FS_AllowDirectPath but I would prefer to not rely on it, due to how easy it might become a security issue.