Rename boolean type to json_bool

In building large systems, there are often clashes over the
preferred base type to use for bool/boolean. At least one
experience has been with a 3rd party proprietary library which
can not be changed. In that case, boolean was a synonym for
unsigned char and used widely in packed structures.

json_object_private.h

@@ -28,7 +28,7 @@ struct json_object
   int _ref_count;
   struct printbuf *_pb;
   union data {
-    boolean c_boolean;
+    json_bool c_boolean;
     double c_double;
     int64_t c_int64;
     struct lh_table *c_object;