Fix typeset attributes -a, -A, -l, -u leaking out of subshells

If an array or upper/lowercase variable was declared with a null
initial value within a virtual/non-forked subshell, like:
	( typeset -a foo; ... )
	( typeset -A foo; ... )
	( typeset -l foo; ... )
	( typeset -u foo; ... )
then the type declaration leaked out of the subshell into the
parent shell environment, though without any values that may
subsequently have been assigned.

src/cmd/ksh93/bltins/typeset.c: setall():
- When deciding whether to create a virtual subshell scope for a
  variable, use sh_assignok(), which was actually designed for the
  purpose, instead of _nv_unset(). This allows getting rid of a
  tangled mess of special-casing that never worked quite right.

src/cmd/ksh93/tests/arrays.sh:
- Add regression tests checking that array declarations don't leak
  out of virtual subshells.

src/cmd/ksh93/tests/attributes.sh:
- Add regression tests for combining the 'export' and 'readonly'
  attributes with every other possible typeset attribute on unset
  variables. This also includes a subshell leak test for each one.

Fixes: https://github.com/ksh93/ksh/issues/88

NEWS

@@ -15,6 +15,9 @@ Any uppercase BUG_* names are modernish shell bug IDs.
   an unrecognized option. 'sleep -: 1' will now show a usage message and
   exit instead of sleep for one second.
 
+- Fixed a bug that caused the 'typeset' variable attributes -a, -A, -l, and
+  -u to leak out of a subshell if they were set without assigning a value.
+
 2020-07-23:
 
 - Fixed an infinite loop that could occur when ksh is the system's /bin/sh.