[NNTP] LIST OVERFIEW.FMT capability

[Urs Janßen]

as CAPABILITIES allows "LIST OVERFIEW.FMT" to be returned but
the indicating capability for "LIST OVERFIEW.FMT" is OVER it might be
usefull to add a note about the OVER cmd (mandatory or not) if
"LIST OVERFIEW.FMT" is advertised.

e.g. is the following (not very usefull) answer allowed

[C] CAPABILITIES
[S] 101 Capability list:
[S] VERSION 2
[S] READER
[S] LIST ACTIVE NEWSGROUPS OVERFIEW.FMT
[S] .

or must OVER be implemented if LIST OVERFIEW.FMT is advertised.