parse-options.[ch]: revert use of "enum" for parse_options()

parse-options.[ch]: revert use of "enum" for parse_options()

Revert the parse_options() prototype change in my recent
352e761388b (parse-options.[ch]: consistently use "enum
parse_opt_result", 2021-10-08) was incorrect. The parse_options()
function returns the number of argc elements that haven't been
processed, not "enum parse_opt_result".

Reported-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>