web/username_prefix.php: no prefix: use $_SERVER["SCRIPT_URL"] instead of rtrimming $_SERVER["REQUEST_URI"]

web/username_prefix.php: has prefix: use same expr whether or not it has a query string, because the with-query-string expr works for both cases

web/username_prefix.php: use if statements instead of `? :` for clarity

web/username_prefix.php: use $subpath instead of $_SERVER["PHP_AUTH_USER"] for clarity

web/.htaccess, username_prefix.php: username-based prefix subpaths: support dotpath in query string (which will be appended verbatim to the prefix). use "__?@__" instead of "__?__&" to signal username-based prefix, so that the query string does not end in a dangling & .

web/.htaccess, web/username_prefix.php: username-based prefix subpaths: support query strings in addition to paths

added web/username_prefix.php, for use with username-based prefix redirects