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