Set the flag via the _mesa_init_gl_program() and NewProgram()
helpers.

In i965 we currently check for the existance of gl_shader_program
to decide if this is an ARB assembly style program or not.

Adding a flag makes the code clearer and will help removes a
dependency on gl_shader_program in the i965 codegen functions.

Also this will allow use to skip initialising sampler units for
linked shaders, we currently memset it to zero again during linking.

Reviewed-by: Eric Anholt <eric@anholt.net>