gkeyfile: Fix C locale handling
As gettext does:
- if C locale is used to set a value, set the untranslated value, i.e. key=value and not key[C]=value;
- if C locale is used to get a value, return the untranslated value, i.e. not the value from key[C]=value, if it ever exists, and do not consider C as an undefined locale otherwise.
Fixes: #3578 (closed)