Support for COMPUTE data sources (CDEF data sources). Removes the RPN

parser and calculator from rrd_graph and puts then in a new file,
rrd_rpncalc.c. Changes to core files rrd_create and rrd_update. Some
clean-up of aberrant behavior stuff, including a bug fix.
Documentation update (rrdcreate.pod, rrdupdate.pod). Change xml format.
-- Jake Brutlag <jakeb@corp.webtv.net>

git-svn-id: svn://svn.oetiker.ch/rrdtool/trunk/program@31 a5681a0c-68f1-0310-ab6d-d61299d08faa