cd - shouldn't ignore $OLDPWD when in a new scope (#249)

This bug was first reported at <https://github.com/att/ast/issues/8>.
The 'cd' command currently takes the value of $OLDPWD from the
wrong scope. In the following example 'cd -' will change the
directory to /bin instead of /tmp:

    $ OLDPWD=/bin ksh93 -c 'OLDPWD=/tmp cd -'
    /bin

src/cmd/ksh93/bltins/cd_pwd.c:
- Use sh_scoped() to obtain the correct value of $OLDPWD.
- Fix a use-after-free bug. Make the 'oldpwd' variable a static
  char that points to freeable memory. Each time cd is used, this
  variable is freed if it points to a freeable memory address and
  isn't also a pointer to shp->pwd.

src/cmd/ksh93/sh/path.c: path_pwd():
- Simplify and add comments.
- Scope $PWD properly.

src/cmd/ksh93/tests/builtins.sh,
src/cmd/ksh93/tests/leaks.sh:
- Backport the ksh2020 regression tests for 'cd -' when $OLDPWD is
  set.
- Add test for $OLDPWD and $PWD after subshare.
- Add test for $PWD after 'cd'.
- Add test for possible memory leak.
- Add testing for 'unset' on OLDPWD and PWD.

src/cmd/ksh93/COMPATIBILITY:
- Add compatibility note about changes to $PWD and $OLDPWD.

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

NEWS

@@ -3,6 +3,14 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2021-03-31:
+
+- Fixed a bug that caused 'cd -' to ignore the current value of $OLDPWD
+  when it's set to a different directory in a new scope.
+
+- Fixed a related bug that caused ksh to use the wrong value for $PWD
+  when in a new scope.
+
 2021-03-29:
 
 - Fixed an intermittent crash that could occur in vi mode when using the 'b'