gallium/dril: Compatibility stub for the legacy DRI loader interface

Following on from !28138 (closed), I wanted to sketch what a minimal DRI stub for Xorg might look like. I haven't tested it even with Xvfb yet, but it compiles, and should give some idea of what we could do. Things I'd like to address before merging:

• test with drisw
• implement and test with dri2
• sanity check the fbconfig format list
• take over the driver symlink farm from the dri target
• think through the meson configuration logic
• possibly a better name
• probably something else

I'd prefer this not to get too complicated though. My main goals are to free Mesa from dri_interface.h internally, and to preserve xserver's ability to initialize GLX at all. Anything beyond that and you really should just upgrade your X server.