GraphicsDisplaySyncAcrossDualHeadsTestLinux: Refine implementation a bit.

-> Use first two assigned display heads of x-screen, not always heads 0 and 1,
   as those might be the wrong ones on a multi x-screen setup.

-> Restore screentohead mapping after testing procedure.

-> Add support for fast AMD multi-display sync before test as syncmethod 1.
-> Move old broken driftsync to syncmethod 2 and warn about its brokeness.
-> Cleanup help texts and plotting code.