Consider making rrd_strtod and rrd_snprintf public library

[hramrach]

Is your feature request related to a problem? Please describe.
rrdtool is not the only project that needs locale-independent string formatting and deformatting

Describe the solution you'd like
A separate librrd_string or something that exposes rrd_strtod and rrd_snprintf functions as public API

Describe alternatives you've considered
Importing some sort of strtod/printf implementation into every project that needs locale independent string functions.

Additional context
Pretty much every tool that feeds data to librrd needs this functionality when written in C.