+20120512
+ + 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
+