[osh-language] Try out different 'unset' behavior.

It matches bash, mksh, and yash now.  Instead of dash/ash or zsh.

Addresses an issue in #653 (ble.sh).

However this makes some tests in test/spec.sh assign fail.  See the "if
0".

core/state.py

@@ -1428,8 +1428,14 @@ def Unset(self, lval, lookup_mode, strict):
 
     with tagswitch(lval) as case:
       if case(lvalue_e.Named):  # unset x
-        name_map[cell_name].val = value.Undef()
-        cell.exported = False
+        if 0:  # flip this
+          # This behavior is good for test/spec.sh assign -r 24-27
+          name_map[cell_name].val = value.Undef()
+          cell.exported = False
+        else:
+          # This behavior is good for test/spec.sh builtin-vars -r 24 (ble.sh)
+          del name_map[cell_name]
+
         # This should never happen because we do recursive lookups of namerefs.
         assert not cell.nameref, cell
 

spec/builtin-vars.test.sh

@@ -326,23 +326,46 @@ foo=
 
 #### Unset and scope (bug #653)
 unlocal() { unset "$@"; }
-check4() {
-  hello=global
 
-  local hello=local
-  echo hello=$hello
+level2() {
+  local hello=yy
 
+  echo level2=$hello
   unlocal hello
-  echo hello=$hello
+  echo level2=$hello
 }
-check4
-## STDOUT:
-hello=local
+
+level1() {
+  local hello=xx
+
+  level2
+
+  echo level1=$hello
+  unlocal hello
+  echo level1=$hello
+
+  level2
+}
+
 hello=global
-## END
-## OK dash/ash/zsh/osh STDOUT:
-hello=local
-hello=
+level1
+
+# bash, mksh, yash agree here.
+## STDOUT:
+level2=yy
+level2=xx
+level1=xx
+level1=global
+level2=yy
+level2=global
+## END
+## OK dash/ash/zsh STDOUT:
+level2=yy
+level2=
+level1=xx
+level1=
+level2=yy
+level2=
 ## END
 
 #### Unset invalid variable name