JSON library doesn't work with locales that use comma as decimal separator

[danpla]

ccan/json library uses locale-dependent C routines (strtod() and sprintf()) to deal with numbers. If the program uses a locale with a comma as a decimal separator, the library will truncate numbers on reading (strtod() will ignore . and everything after it) and create broken JSONs (numbers will have , as separator instead of . as required by the standard).