MINOR: jwt: Improve 'jwt_tokenize' function

MINOR: jwt: Improve 'jwt_tokenize' function

The 'jwt_tokenize' function that can be used to split a JWT token into
its subparts can either fully process the token (from beginning to end)
when we need to check its signature, or only partially when using the
jwt_header_query or jwt_member_query converters. In this case we relied
on the fact that the return value of the 'jwt_tokenize' function was not
checked because a '-1' was returned (which was not actually an error).

In order to make this logic more explicit, the 'jwt_tokenize' function
now has a way to warn the caller that the token was invalid (less
subparts than the specified 'item_num') or that the token was not
processed in full (enough subparts found without parsing the token all
the way).
The function will now only return 0 if we found strictly the same number
of subparts as 'item_num'.