Fix ${x=y} and ${x:=y} for numeric types of x

These POSIX expansions first assign y to x if x is unset or empty,
respectively, and then they yield the value of x. This was not
working on any ksh93 version if x was typeset as numeric (integer
or float) but still unset, as in not assigned a value.

$ unset a; typeset -i a; printf '%q\n' "${a:=42}" "$a"
0
''

Expected output:
42
42

src/cmd/ksh93/sh/macro.c:
- Fix the test for set/unset variable. It was broken because it
  only checked for the existence of the node, which exists after
  'typeset', but did not check if a value had been assigned. This
  additional check needs to be done with the nv_isnull() macro, but
  only for expansions of the regular M_BRACE type. Special
  expansions cannot have an unset state.
- As of commit 95294419, we know that an nv_optimize() call may be
  needed before using nv_isnull() if the shell is compiled with
  SHOPT_OPTIMIZE. Move the nv_optimize() call from that commit
  forward to before the new check that calls nv_isnull(), and only
  bother with it if the type is M_BRACE.

src/cmd/ksh93/tests/variables.sh:
- Add tests for this bug. Test float and integer, and also check
  that ${a=b} and ${a:=b} correctly treat the value of 'b' as an
  arithmetic expression of which the result is assigned to 'a' if
  'a' was typeset as numeric.

src/cmd/ksh93/tests/attributes.sh,
src/cmd/ksh93/tests/comvar.sh,
src/cmd/ksh93/tests/nameref.sh,
src/cmd/ksh93/tests/types.sh:
- Fix a number of tests to report failures correctly.

Resolves: https://github.com/ksh93/ksh/issues/157

src/cmd/ksh93/tests/attributes.sh

@@ -204,7 +204,7 @@ else	b1=iIWTk5ZAppaZk4Q=
 fi
 z=$b1
 typeset -b x=$b1
-[[ $x == "$z" ]] || print -u2 'binary variable not expanding correctly'
+[[ $x == "$z" ]] || err_exit "binary variable not expanding correctly ($(printf %q "$x") != $(printf %q "$z"))"
 [[  $(printf "%B" x) == $t1 ]] || err_exit 'typeset -b not working'
 typeset -b -Z5 a=$b1
 [[  $(printf "%B" a) == $w1 ]] || err_exit 'typeset -b -Z5 not working'