Move plpgsql's fetchArgInfo() into funcapi.c, and rename to

Move plpgsql's fetchArgInfo() into funcapi.c, and rename to
get_func_arg_info() for consistency with other names there.
This code will probably be useful to other PLs when they start to
support OUT parameters, so better to have it in the main backend.
Also, fix plpgsql validator to detect bogus OUT parameters even when
check_function_bodies is off.