Detect when ACS doesn't work

[atagar]

Migrated from ticket 18540.

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

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...