parse-options: adjust parse_opt_unknown_cb()s declared return type

In f41179f (parse-options: avoid magic return codes, 2019-01-27),
the signature of the low-level parse-opt callback function was changed
to return an `enum`.

And while the implementations were changed, one declaration was left
unchanged, still claiming to return `int`.

This can potentially lead to problems, as compilers are free to choose
any integral type for an `enum` as long as it can represent all declared
values.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

parse-options.h

@@ -293,7 +293,9 @@ int parse_opt_commit(const struct option *, const char *, int);
 int parse_opt_tertiary(const struct option *, const char *, int);
 int parse_opt_string_list(const struct option *, const char *, int);
 int parse_opt_noop_cb(const struct option *, const char *, int);
-int parse_opt_unknown_cb(struct parse_opt_ctx_t *ctx, const struct option *, const char *, int);
+enum parse_opt_result parse_opt_unknown_cb(struct parse_opt_ctx_t *ctx,
+					   const struct option *,
+					   const char *, int);
 int parse_opt_passthru(const struct option *, const char *, int);
 int parse_opt_passthru_argv(const struct option *, const char *, int);