i965/pre-Gen6: Don't rely on VERT_RESULT_HPOS <-> FRAG_ATTRIB_WPOS.

At present, the _mesa_vert_result_to_frag_attrib() function maps
VERT_RESULT_HPOS to FRAG_ATTRIB_WPOS.  This is clumsy, since
_mesa_vert_result_to_frag_attrib() is supposed to map vertex outputs
to their corresponding equivalent fragment inputs, and HPOS and WPOS
are in no way equivalent.

Only one piece of code relied on this clumsy correspondence--the
determination of which fragment inputs are present when computing the
urb_setup for pre-Gen6 fragment shaders.  This patch modifies the
urb_setup computation so that it always assumes FRAG_ATTRIB_WPOS is
present (because it always is).
mtypes.h: Stop mapping VERT_RESULT_HPOS <-> FRAG_ATTRIB_WPOS.

Previously, the _mesa_vert_result_to_frag_attrib() and
_mesa_frag_attrib_to_vert_result() functions established a
correspondence between VERT_RESULT_HPOS and FRAG_ATTRIB_WPOS.  This
was bogus, since the former is gl_Position and the latter is
gl_FragCoord, which aren't in any way the same thing.

No code relies on this bogus correspondence anymore, so remove it.
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.
WIP: Remap VERT_RESULT to new enum