Add support for Valve FS to natives that use file handles

Drifter did a large start of this, and I've recently revived it. It's still a bit of a work-in-progress, but most or all feature-complete. (all of the file-related natives should be done. Directory-related natives and LogToOpenFile are untouched).

The goal is to make more operations possible with files via Valve FS since Valve search paths and vpks are being used more and more.
Good idea, but would probably want it both before and after. Return false if it didn't exist or wasn't deleted for whatever reason.

Flush is also like this, always returning true for the Valve filesystem since it is just a void return. That seems less important and without as clear of a workaround.