resolve the "flush inconsistency":

* rrd_flush (a leftover from when rrdtool was using stream based io) is turned into a no-op
  but still exported to preserve library compatibility.

* rrd_cmd_flush (the implementation of "rrdtool flush") is renamed to rrd_flushcached
  along with its command line invocation which is now rrdtool flushcached

--tobi

git-svn-id: svn://svn.oetiker.ch/rrdtool/trunk/program@1812 a5681a0c-68f1-0310-ab6d-d61299d08faa