ZaphodHeads improvements for xf86-video-modesetting

[Mario Kleiner]

These are some additional fixes to go on top of Dave's ZaphodHeads(v2)
patch for the modesetting ddx. Patch 1/2 copy and paste ported from
similar patches already in radeon/amdgpu/nouveau ddx.

Patch 2/4 is copy and paste port of a patch which is pending for
nouveau-ddx, on the mailing list but so far not reviewed. Tested
to fix problems on some nv50 cards.

Plus adding some missing entries in the modesetting man page.

thanks,
-mario