This moves selection of the initial RRA row into the rrd_open.c API

This moves selection of the initial RRA row into the rrd_open.c API

The current implementation (random row) is used by default.  However, it
now provides an opportunity for alternative implementations to integrate
with rrdtool in a single place.

Maybe there are other places in rrdtool where I should insert calls to
the function rrd_notify_row()?

This has been tested with rrdtool create and rrdtool info to verify that
random rows are selected by default (existing behaviour preserved).

-- Daniel Pocock

git-svn-id: svn://svn.oetiker.ch/rrdtool/trunk/program@1606 a5681a0c-68f1-0310-ab6d-d61299d08faa