gallivm,draw,llvmpipe: mass rename of unit->texture_unit/sampler_unit

gallivm,draw,llvmpipe: mass rename of unit->texture_unit/sampler_unit

Make it obvious what "unit" this is (no change in functionality).
draw still uses "unit" in places where it changes the shader by adding
texture sampling itself - it seems like this can't work with shaders
using dx10-style sample opcodes (can't mix gl-style and dx10-style
sample instructions in a shader).

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>