web/username_prefix.php: user2path(): added support for padding used to visually separate 1st element of URL (x@url -> x@url)

bugfix: web/username_prefix.php: need to handle : in login (which would normally indicate a password)

web/.htaccess, username_prefix.php: username-based prefix subpaths: don't require the // at the end , to avoid clutter. IMPORTANT: in order to access the home page after visiting a URL with a username, you must now append "." to the host ("vegpath.org.") (other pages are not affected by this problem). since vegpath.org is a URL shortener, this is a worthwhile tradeoff to avoid needing the // suffixes everywhere.

web/username_prefix.php: $_SERVER["PHP_AUTH_USER"] test: factored $_SERVER["SCRIPT_URL"] out of both branches of if statement

web/username_prefix.php: don't remove @ from query string because that is no longer a special character

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