Use shader_arb_get_dst_param instead of get_register_name to find the register
name. Even though this adds support for modifiers(which aren't allowed by
native), this shouldn't hurt. If an app passes in an incorrect shader it
should be caught in the frontend.

Most of the extra work done in the pixel shader code was not needed or
redundant. This patch unifies the two paths.

Mostly based on the code of pshader_gen_input_modifier_line. The space-adding
behavior of shader_arb_add_src_param was removed because the plurality of
instruction handlers passes an uninitialized buffer in and expects a register
name written to its start, and only map2gl and rcp_rsq use the space-adding
stuff. I'll change rcp_rsq in a later patch anyway. I changed the name to
shader_arb_get_src_param to reflect this behavior.

Make a copy of the string data, and move the function to shader.c.