Don't sync pending flags when there aren't any

Disregard "almost empty" pending flags files.
Mark messages as read when opening them
Reshuffle warn vs message for FETCH responses

Microsoft Exchange is not going anywhere, so let's drop the severity for
UID-less FETCH responses to UID STORE commands a bit.  On the other
hand, we'd want to know if we're ignoring message bodies.
Key binding summary for bic-message-mode docstring