c++: Simplify process_template_parm

c++: Simplify process_template_parm

Process_template_parm ends up walking the parameter list twice.
There's not need to do this.  Just rember the final node and modify
its CHAIN directly.  Also comment on why end_template_parm_list does a
pop and a push, rather than modifying the header in place.

	pt.c (process_template_parm): Don't walk the template list twice,
	remember the final node instead.
	(end_template_parm_list): Refactor.  Comment on why we do a pop
	and a push.