mtypes.h: Add new gl_varying_slot enum, and bitfield defines.

Future patches will make use of the enum.  It will eventually take the
place of the existing enums gl_vert_result, gl_geom_attrib,
gl_geom_result, and gl_frag_attrib, all of which represent essentially
the same information but using inconsistent values.
mtypes.h: Modify gl_vert_result to refer to new gl_varying_slot enum.
Replace gl_vert_result enum with gl_varying_slot.
mtypes.h: Modify gl_geom_attrib to refer to new gl_varying_slot enum.
Replace gl_geom_attrib enum with gl_varying_slot.
mtypes.h: Modify gl_geom_result to refer to new gl_varying_slot enum.
Replace gl_geom_result enum with gl_varying_slot.
mtypes.h: Modify gl_frag_attrib to refer to new gl_varying_slot enum.
Get rid of _mesa_vert_result_to_frag_attrib()
Get rid of _mesa_frag_attrib_to_vert_result()
Replace gl_frag_attrib enum with gl_varying_slot.