+ correct an off-by-one in comparison in waddch_literal() which caused
scrolling when a double-cell character would not fit at the lower
right corner of the screen (report by Benno Schulenberg).
+ split-out att610+cvis, vt220+cvis, vt220+cvis8 -TD
+ add vt220-base, for terminal emulators which generally have not
supported att610's blinking cursor control -TD
+ use vt220+cvis in vt220, etc -TD
+ use att610+cvis, xterm+tmux and ansi+enq in kitty -TD
+ use vt220+cvis in st, terminology, termite since they ignore
blinking-cursor detail in att610+cvis -TD