parse-options: avoid magic return codes

parse-options: avoid magic return codes

Give names to these magic negative numbers. Make parse_opt_ll_cb
return an enum to make clear it can actually control parse_options()
with different return values (parse_opt_cb can too, but nobody needs
it).

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>