[PKT_SCHED]: Rework QoS and/or fair queueing configuration

Make "QoS and/or fair queueing" have its own menu, it's too big to be
inlined into "Network options". Remove the obsolete NET_QOS option.
Automatically select NET_CLS if needed. Do the same for NET_ESTIMATOR
but allow it to be selected manually for statistical purposes. Add
comments to separate queueing from classification. Fix dependencies
and ordering of classifiers. Improve descriptions/help texts and
remove outdated pieces.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>