* rrd_open: rrd_close does not purge file from cache

* rrd_open: rrd_close does not purge file from cache
* rrd_open: new function rrd_dontneed for purging un-needed pages from core
* rrd_open: in linux at least only fadivse DONTNEED has the power to purge
  pages from cache, so letst call madvise as well as fadvise
* rrd_create: uses open/write/close now, flushes file to disk and keeps only hot pages in core
* rrd_update: keeps only hot pages in core
* configure enables FADVISE even when mmap is in use

git-svn-id: svn://svn.oetiker.ch/rrdtool/trunk@1156 a5681a0c-68f1-0310-ab6d-d61299d08faa