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.

NEWS

@@ -3,6 +3,11 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2020-09-30:
+
+- Fixed: 'typeset -xu' and 'typeset -xl' (export + change case) failed to
+  change the case of a variable's value in certain conditions.
+
 2020-09-28:
 
 - While executing a ksh-style function, ksh 93u+ ignored all signals for which