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
since double_str is 12 char you may want to set %11[-0-9.e]  to prevent an overflow (char 12 is the \0)
and maybe go to 19/20 char just to be sure