CDEF operators SHIFT, SQRT, SORT, and REV (reverse). See documentation for what

CDEF operators SHIFT, SQRT, SORT, and REV (reverse).  See documentation for what
they do. This included removal of redundant code in the
rrd_xport path, replaced with a call to rrd_graph_script().
-- David M. Grimes <dgrimes@navisite.com>

git-svn-id: svn://svn.oetiker.ch/rrdtool/trunk/program@290 a5681a0c-68f1-0310-ab6d-d61299d08faa