parse-options.[ch]: consistently use "enum parse_opt_flags"

Use the "enum parse_opt_flags" instead of an "int flags" as arguments
to the various functions in parse-options.c.

Even though this is an enum bitfield there's there's a benefit to
doing this when it comes to the wider C ecosystem. E.g. the GNU
debugger (gdb) will helpfully detect and print out meaningful enum
labels in this case. Here's the output before and after when breaking
in "parse_options()" after invoking "git stash show":

Before:

    (gdb) p flags
    $1 = 9

After:

    (gdb) p flags
    $1 = (PARSE_OPT_KEEP_DASHDASH | PARSE_OPT_KEEP_UNKNOWN)

Of course as noted in[1] there's a limit to this smartness,
i.e. manually setting it with unrelated enum labels won't be
caught. There are some third-party extensions to do more exhaustive
checking[2], perhaps we'll be able to make use of them sooner than
later.

We've also got prior art using this pattern in the codebase. See
e.g. "enum bloom_filter_computed" added in 312cff52074 (bloom: split
'get_bloom_filter()' in two, 2020-09-16) and the "permitted" enum
added in ce910287e72 (add -p: fix checking of user input, 2020-08-17).

1. https://lore.kernel.org/git/87mtnvvj3c.fsf@evledraar.gmail.com/
2. https://github.com/sinelaw/elfs-clang-plugins/blob/master/enums_conversion/README.md

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

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