mesa/sso: rename Shader to the pointer _Shader

mesa/sso: rename Shader to the pointer _Shader

Basically a sed but shaderapi.c and get.c.
get.c => GL_CURRENT_PROGAM always refer to the "old" UseProgram behavior
shaderapi.c => the old api stil update the Shader object directly

V2: formatting improvement

V3 (idr):
* Rebase fixes after a block of code was moved from ir_to_mesa.cpp to
  shaderapi.c.
* Trivial reformatting.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>