Fix bugs


Add checks when setting DataSources that the requested iface
is indeed of the right type with respect to the RMON group that is
using it. For example, etherStats should and will return a BAD VALUE
when the DataSource is set to a TokenRing iface.


Resolve ambiguities regarding RFC1271


Filter OctetsRequested Change

    If this variable is changed the whole buffer is copied to a
    new one, even those entries that should be deleted. They will
    be deleted if they get overwritten by newer entries during the
    copying. This is the simplest method but also the most
    expensive one, especially if you are going from a large to a small
    buffer where most of the older entries will be overwritten anyway.

