Save $? when discipline triggered without command (#226)

A discipline function could incorrectly influence the value of $?
(exit status of last command) outside its context if it was
triggered without another command being run, e.g. when a prompt
variable is read, or COLUMNS or LINES is set.

Reproducers include:

PS1 prompt:

    $ PS1.get() { true; }
    $ false
    $ echo $?
    0

PS2 prompt:

    $ PS2.get() { return 13; }
    $ \
    > 
    $ echo $?
    13

The set discipline is affected too, e.g. COLUMNS and LINES:

    $ COLUMNS.set() { return 13; }
    $ true
    $ (press return)
    $ echo $?
    13

There are probably other contexts where the shell reads or changes
variables without running commands, allowing their get or set
disciplines to influence $?. So this commit makes ksh save $? for
all .get, .set, .append, and .unset discipline calls.

src/cmd/ksh93/sh/nvdisc.c:
- assign(): Save/restore $? when running a .set/.append/.unset
  discipline function.
- lookup(): Save/restore $? when running a .get discipline.

src/cmd/ksh93/tests/pty.sh:
- Add a regression test for $? after displaying a prompt
  and when setting a LINES.set discipline function.

src/cmd/ksh93/tests/return.sh:
- The above test fails in script form on ksh93u+ and ksh2020, as
  it exposes another form of #117 that occurs after running a
  subshell. Add the above regression test here as well
  (re: 092b90da).

Co-authored-by: Martijn Dekker <martijn@inlv.org>

NEWS

@@ -3,6 +3,13 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2021-03-16:
+
+- Fixed a bug in interactive shells: if a variable used by the shell called
+  a discipline function (such as PS1.get() or COLUMNS.set()), the value of $?
+  was set to the exit status of the discipline function instead of the last
+  command run.
+
 2021-03-15:
 
 - If the HOME variable is unset, the bare tilde ~ now expands to the current