Fix test -v for numeric types & set/unset state for short int

This commit fixes two interrelated problems.

1. The -v unary test/[/[[ operator is documented to test if a
   variable is set. However, it always returns true for variable
   names with a numeric attribute, even if the variable has not
   been given a value. Reproducer:
	$ ksh -o nounset -c 'typeset -i n; [[ -v n ]] && echo $n'
	ksh: n: parameter not set
   That is clearly wrong; 'echo $n' should never be reached and the
   error should not occur, and does not occur on mksh or bash.

2. Fixing the previous problem revealed serious breakage in short
   integer type variables that was being masked. After applying
   that fix and then executing 'typeset -si var=0':
   - The conditional assignment expansions ${var=123} and
     ${var:=123} assigned 123 to var, even though it was set to 0.
   - The expansions ${var+s} and ${var:+n} incorrectly acted as if
     the variable was unset and empty, respectively.
   - '[[ -v var ]]' and 'test -v var' incorrectly returned false.
   The problems were caused by a different storage method for short
   ints. Their values were stored directly in the 'union Value'
   member of the Namval_t struct, instead of allocated on the stack
   and referred to by a pointer, as regular integers and all other
   types do. This inherently broke nv_isnull() as this leaves no
   way to distinguish between a zero value and no value at all.
   (I'm also pretty sure it's undefined behaviour in C to check for
   a null pointer at the address where a short int is stored.)
   The fix is to store short ints like other variables and refer
   to them by pointers. The NV_INT16P combined bit mask already
   existed for this, but nv_putval() did not yet support it.

src/cmd/ksh93/bltins/test.c: test_unop():
- Fix problem 1. For -v, only check nv_isnull() and do not check
  for the NV_INTEGER attribute (which, by the way, is also used
  for float variables by combining it with other bits).
  See also 5aba0c72 where we recently fixed nv_isnull() to
  work properly for all variable types including short ints.

src/cmd/ksh93/sh/name.c: nv_putval():
- Fix problem 2, part 1. Add support for NV_INT16P. The code is
  simply copied and adapted from the code for regular integers, a
  few lines further on. The regular NV_SHORT code is kept as this
  is still used for some special variables like ${.sh.level}.

src/cmd/ksh93/bltins/typeset.c: b_typeset():
- Fix problem 2, part 2. Use NV_INT16P instead of NV_SHORT.

src/cmd/ksh93/tests/attributes.sh:
- Add set/unset/empty/nonempty tests for all numeric types.

src/cmd/ksh93/tests/bracket.sh,
src/cmd/ksh93/tests/comvar.sh:
- Update a couple of existing tests.
- Add test for [[ -v var ]] and [[ -n ${var+s} ]] on unset
  and empty variables with many attributes.

src/cmd/ksh93/COMPATIBILITY:
- Add a note detailing the change to test -v.

src/cmd/ksh93/data/builtins.c,
src/cmd/ksh93/sh.1:
- Correct 'typeset -C' documentation. Variables declared as
  compound are *not* initially unset, but initially have the empty
  compound value. 'typeset' outputs them as:
	typeset -C foo=()
  and not:
	typeset -C foo
  and nv_isnull() is never true for them. This may or may not
  technically be a bug. I don't think it's worth changing, but
  it should at least be documented correctly.

NEWS

@@ -8,6 +8,11 @@ Any uppercase BUG_* names are modernish shell bug IDs.
 - The ${!foo@} and ${!foo*} expansions yield variable names beginning with foo,
   but excluded 'foo' itself. The fix for this is now backported from 93v- beta.
 
+- test -v var, [ -v var ], and [[ -v var ]] did not correctly test if a
+  variable is set or unset after it has been given a numeric attribute with
+  'typeset' but not yet assigned a value. This has been fixed so that
+  [[ -v var ]] is now equivalent to [[ -n ${var+set} ]] as documented.
+
 2021-03-07:
 
 - Fixed the typeset -p display of short integers without an assigned value.