Fix 'typeset -p' output of compound array types (#453)

This bugfix was backported from ksh93v- 2012-10-04. The bug fixed
by this change is one that causes 'typeset -p' to omit the -C flag
when listing compound arrays belonging to a type:

   $ typeset -T Foo_t=(compound -a bar)
   $ Foo_t baz
   $ typeset -p baz.bar
   typeset -a baz.bar=''  # This should be 'typeset -C -a'

src/cmd/ksh93/sh/nvtype.c:
- Backport change from 93v- 2012-10-04 that sets the array nvalue to
  a pointer named Null (which is "") in nv_mktype(), then to Empty
  in fixnode().
- Change the Null name from the 93v- code to AltEmpty to avoid
  misleading code readers into thinking that it's a null pointer.

src/cmd/ksh93/tests/types.sh:
- Backport the relevant 93v- changes to the types regression tests.

Co-authored-by: Martijn Dekker <martijn@inlv.org>

NEWS

@@ -14,10 +14,13 @@ Any uppercase BUG_* names are modernish shell bug IDs.
   an incompletely defined 'foo_t' built-in comamnd that will crash the shell
   when used. Instead, it is now silently ignored for backwards compatibility.
 
-- A bug in which the output of a two dimensional sparse indexed array would
-  cause the second subscript to be treated as an associative array when read
-  back in has been fixed. Elements that are sparse indexed arrays are now
-  prefixed with "typeset -a".
+- Compound arrays belonging to a type created with 'typeset -T' now have
+  their -C attribute preserved in the output of 'typeset -p'.
+
+- A bug has been fixed in which the 'typeset -p' output of a two-dimensional
+  sparse indexed array would cause the second subscript to be treated as an
+  associative array when read back in by the shell. Elements that are sparse
+  indexed arrays are now prefixed with "typeset -a".
 
 2022-02-05: