new function rrd_fetch_r() (and make the

new function rrd_fetch_r() (and make the
strings const char* instead of char*).  The only difference between
rrd_fetch_r() and rrd_fetch_fn() is that rrd_fetch_r() receives the
consolidation function as a string (instead of an enum cf_en) and is thread-safe -- Sam Umbach

git-svn-id: svn://svn.oetiker.ch/rrdtool/branches/1.2/program@1028 a5681a0c-68f1-0310-ab6d-d61299d08faa