parse-options.h: make the "flags" in "struct option" an enum

parse-options.h: make the "flags" in "struct option" an enum

Change the "flags" members of "struct option" to refer to their
corresponding "enum" defined earlier in the file.

The benefit of changing this to an enum isn't as great as with some
"enum parse_opt_type" as we'll always check this as a bitfield, so we
can't rely on the compiler checking "case" arms for us. But let's do
it for consistency with the rest of the file.

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