Adds two drreg features to help support separate control flow paths such as a slowpath and a fastpath: drreg_reservation_info_ex(), which provides information on registers which have been unreserved but not yet lazily restored, and drreg_statelessly_restore_app_value() which restores app state without changing drreg state, to retain parity with a separate path.
Adds some tests of the new features.
Issue: #511