The current implementation of colwidth was treating 'A'mbiguous
characters as wide, which was incorrect in a non-East Asian context.
As per http://unicode.org/reports/tr11/#Recommendations, we should
instead default to 'narrow' if we don't know better. As character
width is dependent on the particular font used and we have no idea
what fonts are in use, this recommendation applies.
This introduces HGENCODINGAMBIGUOUS to get the old behavior back.
