New _mesa_debug() function to replace fprintf() calls.

New _mesa_debug() function to replace fprintf() calls.
Some source files updated to call _mesa_debug(), but not finished.
Added __GLimports as a parameter to _mesa_create/init_context() and
updated drivers accordingly.
Fleshed-out more of the __GLimports and __GLexports functionality.
Removed run-time config file support (config.c)