Detect when ACS doesn't work

Probably the most common annoying visual bug I've seen is broken curses ACS support. This causes us to show 'mwqqqqqqqqqqqqq' for window borders...

https://www.atagar.com/transfer/tmp/acs_error.png

We can sidestep this by showing ASCII pipes instead ('+--------+'), but to do that we need to be able to detect when ACS is broken. Thus far I'm not sure how to do this - tips welcome...