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>

src/cmd/ksh93/COMPATIBILITY

@@ -111,6 +111,12 @@ For more details, see the NEWS file and for complete details, see the git log.
 	Turning on the new --globcasedetect shell option restores
 	case-insensitive globbing for case-insensitive file systems.
 
+22.	If $PWD or $OLDPWD are passed as invocation-local assignments to cd,
+	their values are no longer altered in the outer scope when cd finishes.
+	For example:
+		ksh -c 'OLDPWD=/bin; OLDPWD=/tmp cd - > /dev/null; echo $OLDPWD'
+		ksh -c 'cd /var; PWD=/tmp cd /usr; echo $PWD'
+	now prints '/bin' followed by '/var'.
 ____________________________________________________________________________
 
 		KSH-93 VS. KSH-88