prune xrefs to xorgconfig

mibstore.h and its miInitializeBackingStore() have been otherwise empty
since at least xorg-server 1.10. remove all reference to them as the
header is gone in xorg-server 1.18.

fix the previous - use GET_ABI_MAJOR(ABI_VIDEODRV_VERSION) > 6.

copy a hack from the vmware driver, and avoid using xf86LoaderReqSymLists()
and LoaderRefSymLists() in xorg-server >= 1.7.

xf86Version.h is no more

initial import of xf86-video-imstt-1.1.0