Improve HTTP::getSelfURL() path detection and URL reconstruction

HTTP::getSelfURL() mis-reconstructs the current URL when SimpleSAMLphp is accessed through a rewritten path such as RewriteRule ^/cas/login(.*) /${SSP_APACHE_ALIAS}module.php/casserver/login.php$1 [PT]. In this scenario the public URL is /cas/login?service=..., while Apache internally rewrites it to /simplesaml/module.php/casserver/login.php?service=.... The old implementation assumes that the script-relative filesystem path (for example module.php/casserver/login.php) appears verbatim in $_SERVER['REQUEST_URI'], searches for that substring in the entire REQUEST_URI, and rebuilds the URL by stitching together getBaseURL(), that matched path, and the remainder of REQUEST_URI. With mod_rewrite, REQUEST_URI contains only the external path (/cas/login?...), so the expected script path is not present, or may only appear by accident inside query parameters. This causes getSelfURL() to either fall back to an incorrect URL or to construct one that points to module.php/... instead of /cas/login, or that has a malformed path/query combination, breaking redirects and return URLs that rely on getSelfURL() to reflect the actual public URL.