lei q: support vmd for external-only messages

lei q: support vmd for external-only messages

"lei q" now preserves changes per-message keywords across
invocations when it's --output (Maildir or mbox) is reused
(with or without --augment).

In the future, these changes will be monitored via inotify,
EVFILT_VNODE or IMAP IDLE, too.

Unfortunately, this currently prevents "lei import" from ever
importing a message that's in an external.  That will be fixed
in a future change.