Programs that follow Digital's recommendations will follow ESC(0 -- to

load the DEC Special Graphics (VT line draw) characters into graphics set
GS0 -- with lock shift 0, aka LS0, aka SO, aka ^0 -- to then enable GS0.
Since GS0 is the default, the full procedure is really only necessary when
displaying line draw and alphanumeric characters on the same line. With the
last patch to enable line draw characters, ESC(0 was always taken to mean
ASCII SI (shift in), and ^0 was always taken to to mean SO (shift out),
so for programs that did the right thing (e.g. "pstree"), "dt" was simply
setting, then unsetting, the alternate character set attribute, causing
the desired line draw characters never to be displayed.

To deal with that, introduce a layer of abstraction, such that ESC(0
and friends now only set per-VT variables, rather than directly set the
attribute. Initialize these variables so that SO and SI, taken alone, will
do the expected thing, and extend the state machine so that a changed GS0
will become active at the next carriage return, or immediately if an LS0
was already seen on that line (all very much like a real VT).

Bump version to dt-1.1.7nb2.

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