r1885: tighten the cache consistency with the ntprinters.tdb entry an the in memory...

r1885: tighten the cache consistency with the ntprinters.tdb entry an the in memory cache associated with open printer handles; also make sure that register_messages_flags() doesn't overwrite the originally registers flags
(This used to be commit 540daf71d8ad189af5dd6d45aa1ce2b3d67da752)