r300g: import the last bits of libdrm and cleanup the whole thing

r300g: import the last bits of libdrm and cleanup the whole thing

Based on Dave's branch.

The majority of this commit is a cleanup, mainly renaming things.
There wasn't much code to import, just ioctl calls.

Also done:
- implemented unsynchronized bo_map (important optimization!)
- radeon_bo_is_referenced_by_cs is no longer a refcount hack
- dropped the libdrm_radeon dependency

I'm surprised that this has resulted in less code in the end.