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/include/version.h

@@ -20,7 +20,7 @@
 
 #define SH_RELEASE_FORK	"93u+m"		/* only change if you develop a new ksh93 fork */
 #define SH_RELEASE_SVER	"1.0.0-alpha"	/* semantic version number: https://semver.org */
-#define SH_RELEASE_DATE	"2021-03-29"	/* must be in this format for $((.sh.version)) */
+#define SH_RELEASE_DATE	"2021-03-31"	/* must be in this format for $((.sh.version)) */
 #define SH_RELEASE_CPYR	"(c) 2020-2021 Contributors to ksh " SH_RELEASE_FORK
 
 /* Scripts sometimes field-split ${.sh.version}, so don't change amount of whitespace. */