Add the option to pass a custom path for the multilib config files
    
    The default is /usr/share/pungi/multilib/, pass --multilibconf to
    override this.
    
    This also adds multilib.init() so that an import of multilib doesn't
    immediately setup the classes.
    
    (cherry picked from commit 234524296fd53871aed64690cf6a7d5849ca154a)