Standardize include guards to use OPENVDS_ prefix
Standardize include guards to use OPENVDS_ prefix so they won't get mixed up with other projects (e.g. EXCEPTIONS_H was very generic and asking for trouble, which is now OPENVDS_EXCEPTIONS_H).
Standardize include guards to use OPENVDS_ prefix so they won't get mixed up with other projects (e.g. EXCEPTIONS_H was very generic and asking for trouble, which is now OPENVDS_EXCEPTIONS_H).