Teach the index when it is "dirty", and has unsaved changes.  Consider
the index dirty whenever a caller has added or removed an entry from the
main index, REUC or NAME section, including when the index is completely
cleared.  Similarly, consider the index _not_ dirty immediately after it
is written, or when it is read from the on-disk index.

This allows us to ensure that unsaved changes are not lost when we
automatically refresh the index.