Actually deactivate CDPATH when unsetting it

After 'unset CDPATH', CDPATH continued to work as if nothing
happened. Unsetting it should be a valid way to deactivate it.

This bug is in every ksh93 version.

src/cmd/ksh93/bltins/cd_pwd.c: b_cd():
- Fix a manifest logic error: first check if CDPATH (CDPNOD) is
  unset before assigning to 'cdpath', not the other way around.
  Setting the 'cdpath' pointer is what activates the CDPATH search.

src/cmd/ksh93/tests/variables.sh

@@ -305,6 +305,8 @@ x=$(cd ${tmp#/})
 if	[[ $x != $tmp ]]
 then	err_exit "CDPATH ${tmp#/} does not display new directory"
 fi
+unset CDPATH
+cd "${tmp#/}" >/dev/null 2>&1 && err_exit "CDPATH not deactivated after unset"
 cd "$tmp" || exit
 TMOUT=100
 (TMOUT=20)