Add ne_get_response_location(). Set descriptive errors for 3xx responses with a Location header.
* src/ne_request.c (ne_get_response_location): New function.
(ne_end_request): Set descriptive error for a redirect.
* src/ne_redirect.c (create, post_send, free_redirect, ne_redirect_register): Rewritten as a simple cache of the URI returned by ne_get_response_location. Handle any 3xx response.
* src/ne_redirect.h: Note API is deprecated.
* src/ne_request.h: Describe ne_get_response_location.
* test/request.c (redirect_error): New test.
* test/redirect.c (serve_redir): Removed. (check_redir, redirects): Simplify, and enhance to test ne_get_response_location() directly. Add more relative URI checks and fragment handling.