Fix 'typeset -xu', 'typeset -xl' (rhbz#1188377)

'typeset -xu' and 'typeset -xl' would export the variable but fail
to change case in the value under certain conditions.

Original patch:
642af4d6/f/ksh-20120801-xufix.patch

This applies the patch essentially without change and adds a
regression test based on the reproducer provided in the RH bug.

Unfortunately there is no description of how the patch works and
it's a little obscure to me. As far as I can figure out, the cause
of the problem was that nv_newattr() erroneously processed a
nonexistent size option-argument such as what can be given to
options like typeset -F, e.g. typeset -F3 for 3 digits after the
dot. A nonexistent size argument is represented by the value of -1.

src/cmd/ksh93/include/version.h

@@ -17,4 +17,4 @@
 *                  David Korn <dgk@research.att.com>                   *
 *                                                                      *
 ***********************************************************************/
-#define SH_RELEASE	"93u+m 2020-09-28"
+#define SH_RELEASE	"93u+m 2020-09-30"