[NNTP] LIST OVERFIEW.FMT capability

[Clive D.W. Feather]

Russ Allbery said:
>> Consider an implementation that provides XOVER but not OVER. It can
>> advertise LIST OVERVIEW.FMT but can't advertise OVER.
> 
> Although note that the LIST OVERVIEW.FMT command description says that it
> returns the fields returned by OVER,

Not quite: it says "The information contains one line per field in the
order they are returned by the OVER command". If there is no OVER command,
then the statement means nothing.

However, perhaps we should put "(if implemented)" in there.

Similarly for LIST HEADERS.

> so it's not at all clear what it
> should return on a server that doesn't implement OVER.  It's kind of a
> nonsensical thing to do.

No, because of XOVER.

Note that we are clear that this situation is allowed: see 3.4:

    If the label is not included, the server MAY support none or some of
    the commands, but SHOULD NOT support all of them. In general, there
    will be no way for a client to determine which commands are supported
    without trying them.

("label" is a capability label; the "OVER" label in this instance). Also
note that it says:

    Otherwise the command MUST be fully implemented as specified; a server
    MUST NOT only partially implement any of the commands in this
    specification.

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