eina: add locale-independent eina_convert_strtod_c function

eina: add locale-independent eina_convert_strtod_c function

strtod's behavior is changed by system locale.
http://man7.org/linux/man-pages/man3/strtod.3.html
https://en.wikipedia.org/wiki/Decimal_separator

Because of this, strtod(0.5) returns 0.0 in some locales.
When a given value string is locale-independent, strtod has to be
replaced to eina_convert_strtod_c function.
Internally, it calls strtod_l function with "C" locale.

@feature

Reviewed-by: Cedric BAIL <cedric.bail@free.fr>
Reviewed-by: Vincent Torri <vincent.torri@gmail.com>
Differential Revision: https://phab.enlightenment.org/D6644