Fix doubly applied basepaths to lockfiles.

I have

    define('LOCK_DIRECTORY', '/var/lib/tt-rss');

in `config.php`.

I got some

    fopen(/var/lib/tt-rss//var/lib/tt-rss/update_daemon.lock): failed to open stream: No such file or directory

and

    fopen(/var/lib/tt-rss//var/lib/tt-rss/update.lock): failed to open stream: No such file or directory

messages in the error log, referencing `include/functions.php:961` where the `file_is_locked($filename)` function is defined. That function checks if `LOCK_DIRECTORY . "/$filename"` is readable. In the call in `include/rssfuncs.php`, this function is called with an argument that already has prepended the `LOCK_DIRECTORY`.

This could be fixed in many ways. The `basename()` function that I used should work quite generally and not give trouble with multibyte characters (in a perfect world…) and other things.