Ticket 48872 - Fix segfault and use after free in plugin shutdown
    
    Bug Description:   Plugin_closeall would free the plugin password name scheme
    value *even* if the plugin was not a password scheme.
    
    backend manager would call be_cleanup on the plugin *after* plugin_closeall was
    run. This means plugin_closeall now does not close backends, and we call
    plugin_free from the backend to do this.
    
    Fix Description:  Check the union type to make sure we free the pwdscheme
    only on password plugins.
    
    Expose plugin_free, and call it from the backend manager. Don't close backends
    during the call to plugin_closeall.
    
    https://fedorahosted.org/389/ticket/48872
    
    Author: wibrown
    
    Review by: mreynolds (Thanks!)