Removed setlocale instances https://github.com/oetiker/rrdtool-1.x/issues/504

This branch does the following:
1) Removes instances of setlocale
2) Added rrd_strtod function, which is a locale independent version of strtod function 
3) Made most of the code to use rrd_strtod & rrd_snprintf instead of strtod, atof & snprintf
I would put the double_str[12] on a separate line ... since it is not related to vname at all.