interp: allow unsetting global vars from inside a function again

In dc2e11e "unset" of a global var inside a function was broken,
as the special handling of "export" or "readonly" in function scope
introduced in that commit also incorrectly captured "unset",
calling "parent.Set" with the previous value.

Tweak the logic so that "unset" is handled like it did before,
which fixes the regression.

Fixes #806

interp/interp_test.go

@@ -2381,6 +2381,10 @@ set +o pipefail
 		"x=after\nbefore\n",
 	},
 
+	// unset global from inside function
+	{"f() { unset foo; echo $foo; }; foo=bar; f", "\n"},
+	{"f() { unset foo; }; foo=bar; f; echo $foo", "\n"},
+
 	// name references
 	{"declare -n foo=bar; bar=etc; [[ -R foo ]]", ""},
 	{"declare -n foo=bar; bar=etc; [ -R foo ]", ""},

interp/vars.go

@@ -31,17 +31,21 @@ func (o *overlayEnviron) Get(name string) expand.Variable {
 }
 
 func (o *overlayEnviron) Set(name string, vr expand.Variable) error {
-	// "foo=bar" in a function updates the global scope, unless the variable
-	// has been declared as local.
+	// Manipulation of a global var inside a function
 	if o.funcScope && !vr.Local && !o.values[name].Local {
+		// "foo=bar" on a global var in a function updates the global scope
+		if vr.IsSet() {
+			return o.parent.(expand.WriteEnviron).Set(name, vr)
+		}
 		// "foo=bar" followed by "export foo" or "readonly foo"
-		if !vr.IsSet() {
+		if vr.Exported || vr.ReadOnly {
 			prev := o.Get(name)
 			prev.Exported = prev.Exported || vr.Exported
 			prev.ReadOnly = prev.ReadOnly || vr.ReadOnly
 			vr = prev
+			return o.parent.(expand.WriteEnviron).Set(name, vr)
 		}
-		return o.parent.(expand.WriteEnviron).Set(name, vr)
+		// "unset" is handled below
 	}
 
 	prev := o.Get(name)