Tweaked cylc-list logic.

The changes in cylc/cylc#336 mean that `-t` will no longer work under a `LANG=C` environment rather than when `-t` AND `-b` are used.

This change tweaks the logic so `cylc list -t` will work again under a `LANG=C` environment but will correctly give a warning and disable tree if `cylc list -t -b` is used.