NJS: variable access support

This commit introduces the 'vars' JavaScript object to NJS,
enabling direct access to native variables such as $uri and $arg_foo.
The syntax is `${vars.var_name}` or `${'vars[var_name]'}`.

For example:
    {
        "action": {
            "share": "`/www/html${vars.uri}`"
        }
    }

docs/changes.xml

@@ -65,6 +65,12 @@ conditional access logging.
 </para>
 </change>
 
+<change type="feature">
+<para>
+NJS variables access.
+</para>
+</change>
+
 <change type="feature">
 <para>
 $request_id variable contains a string that is formed using random data and