r300: Add radeon_program and trivial refactoring of r300_fragprog to use it

r300: Add radeon_program and trivial refactoring of r300_fragprog to use it

The idea/hope is that radeon_program will serve as an intermediate
representation for r3xx up to r6xx fragment and vertex programs.
Right now, it is nothing more than a simplistic wrapper around Mesa's
prog_instruction, together with the notion of clauses, taken from r6xx docs.

The clauses will eventually be used to represent the nodes that are used in
r300 family fragment programs.