This patch introduces a feature whereby rrdcached will disallow updates

This patch introduces a feature whereby rrdcached will disallow updates
that do not advance the update time.  This prevents the updates from being
discarded later by rrd_update_r.

This patch attempts to make the most of the protocol's limited ability to
return error text when using a -1 return code.

-- kevin brintnall

git-svn-id: svn://svn.oetiker.ch/rrdtool/trunk/program@1596 a5681a0c-68f1-0310-ab6d-d61299d08faa