* geturl.cc (get_url_to_file): Add parameter `allow_ftp_auth'.

    * geturl.cc (get_url_to_file): Add parameter `allow_ftp_auth'.
        * geturl.h: Change declaration of `get_url_to_file' accordingly.
        * netio.cc (NetIO::NetIO): Add parameter `allow_ftp_auth'.
        Set member `ftp_auth'.
        (NetIO::open): Add parameter `allow_ftp_auth'. Use it in calls
        to constructors of derived classes.
        (NetIO::get_ftp_auth): New method.
        * netio.h: Change class definition accordingly.
        * nio-ftp.cc (NetIO_FTP::NetIO_FTP): Add parameter `allow_ftp_auth'.
        Take global variables `net_ftp_user' and `net_ftp_passwd' into account.
        Add code for ftp authentication retry.
        * nio-ftp.h: Change class definition accordingly.
        * nio-http.cc (NetIO_HTTP::NetIO_HTTP): Add parameter `allow_ftp_auth'.
        Add code for ftp authentication retry in case of proxy connection.
        * nio-http.h: Change class definition accordingly.
        * res.rc: Add FTP authentication dialog.
        * resource.h: Add new constants used in res.rc.
        * state.h: Add variables `net_ftp_user' and `net_ftp_passwd'.