vttest: update to 20210210

20210210
        + improve test-screen for VT420 cursor movement, when left/right
          margins are enabled (prompted by discussion with Tom Szilagyi).

+ updated/improved configure macros, i.e., for shellcheck-warnings

20201225
+ correct highlighting for ISO Greek, Hebrew and Latin-5.

+ correct highlighting for DEC Cyrillic, Greek and Hebrew.

+ add Latin-2 character set for VT510.

+ updated/improved configure macros, i.e., for compiler-warnings

+ update config.guess, config.sub

(nia)