Factor out username/password lookup for reuse

    Update Job is now reduced to checking the state of the
    job and calling the new function get_authentication if
    authentication is required. get_authentication handles
    the keyring integration and takes care of presenting the
    authentication dialog to the user if necessary.
Add show_dialog option to get_authentication

    This allows to silently check if the authentication credentials
    can be found in the keyring and only shows the interactive
    password dialog when running as applet.
Use keyring authentication also from menu

    When you use the authenticate menu option for a job this
    now uses the get_authentication method to look up / save
    credentials also in the gnome keyring if the user requests this.
Fixed man pages make rule to work with make -j.
Actually create the .stamp file (fix for last commit).
...and remove the .stamp file when cleaning.
Use configure.ac's new name when checking for missing translations.
Makefile fixes for udevdir/udevrulesdir.