Reworked multi headers to use linked lists. Multi headers are now using linked lists instead of arrays. Notably, the following fields were changed: r->headers_in.cookies (renamed to r->headers_in.cookie), r->headers_in.x_forwarded_for, r->headers_out.cache_control, r->headers_out.link, u->headers_in.cache_control u->headers_in.cookies (renamed to u->headers_in.set_cookie). The r->headers_in.cookies and u->headers_in.cookies fields were renamed to r->headers_in.cookie and u->headers_in.set_cookie to match header names. The ngx_http_parse_multi_header_lines() and ngx_http_parse_set_cookie_lines() functions were changed accordingly. With this change, multi headers are now essentially equivalent to normal headers, and following changes will further make them equivalent.