Fix day difference count in bic-mailbox--format-date

It was the wrong way around, and thus never triggered the "more than one
year" case.
In state :existing, only react to events for our own connection
Declare more arguments as unused

Ensure that real warnings stick out in the compiler output.
Handle sentinel events in all states
Rename bic--expand-literals to bic-expand-literals

It could potentially be useful for external use; no point in
discouraging that.
Don't get "stuck" if there are no new messages to download

The :download-messages task would fail to report that it finished if
the SEARCH command revealed that we're not interested in downloading
anything.
Flag changes in selected mailbox go through "pending flags"

There is no reliable way of telling exactly what we're doing to the
connection apart from going through the task list.