Wrote api for rrd_strtod

rrd_strtoding makes error handling easier.
It is also for avoiding code repetetion where rrd_strtod has been called
I would suggest to make it so, that you use a local endptr anyway and then if the external endptr is not NULL assign it at the end ... so you get full error reporting regardless

at the moment the code will fail if rrd_strtoding called with NULL endptr