Revert "Sys_DefaultBasePath no longer exists @redsaurus, not even on windows."

This reverts commit 37b78893128fa392b4a6308a8e2d782cf565649d.
I apologize @redsaurus, apparently this does still exist on windows and needed to be re-added to qcommon.h.
We should unify this code with whats in MP though, same with the main() funcs.