SDL 2.0

This is first preview of the SDL 2.0 based code. The binary for Windows can be downloaded here:

http://sobomax.sippysoft.com/digger-201412281449-win32.zip

digger-201412281449: For whatever reason (which escapes me now), I've added FreeBSD/VGL

keyboard deinit/reinit into the SDL timing code 12 years ago, so that
if the SDL sound init fails, we are left with the non-working VGL
keyboard. Remove that code for good now.