sane.ds: Add support for DG_IMAGE/DAT_IMAGELAYOUT/GET and SET. Enables Acrobat to use custom scan sizes.

sane.ds: Remove large chunks of stubbed optional functionality; if it's optional, we don't have to support it, and it is not a stub.

sane.ds: More correctly detect an end of scan job from sane; this enables Acrobat to pull multiple pages in one scan.

This change was triggered because of a bug in libsane-hpaio.so, namely
that it relies on libsane.so being dlopen'd RTLD_GLOBAL so that
sanei_init_debug can be resolved.

Autoprobing via DG_CONTROL/DAT_IDENTITY/MSG_GET.

Implement the DG_IMAGE/DAT_IMAGEMEMXFER/MSG_GET code path. Along with
a number of corrections and improvements to allow the actual aquiring
of images to succeed.

Marcus Meissner <marcus@jet.franken.de>
Merged into WineHQ, made sure it works without SANE.