Final pass of libdrm.so work:

- Add $(DESTDIR) for distributors doing package creation
- Remove OS-specific include path from build
- Add /usr/include/drm for driver-kernel API
- Install all of shared-core/*.h in /usr/include/drm
- Rename xf86drm.h to libdrm.h since we're not X biased anymore
- Include backwards compat for xf86drm.h name, with a warning
- Fix libdrm source to account for drm.h living in /usr/include/drm