Wrote api for rrd_strtod

rrd_strtoding makes error handling easier.
It is also for avoiding code repetetion where rrd_strtod has been called
passing endptr as 0 is fine but you may want to check the return code from rrd_strtoding so that you can
return if something wrong:

if (rrd_strtoding(..) != 2){
   return;
}