+ corrected 'op' for bterm (report by Samuel Thibault) -TD
+ modify test/background.c to demonstrate a background character
holding a colored ACS_HLINE. The behavior differs from SVr4 due to
the thick- and double-line extension (cf: 20091003).
+ modify handling of acs characters in PutAttrChar to avoid mapping an
unmapped character to a space with A_ALTCHARSET set.
+ rewrite vt520 entry based on vt420 -TD