fix(brctl): use available interfaces instead of configured

When adding a network interface to a bridge we most probably want to
use one that hasn't been configured yet.

Author: steelman

completions/brctl

@@ -26,7 +26,7 @@ _brctl()
         3)
             case $command in
                 addif | delif)
-                    _configured_interfaces
+                    _available_interfaces
                     ;;
                 stp)
                     COMPREPLY=($(compgen -W 'on off' -- "$cur"))