w(1): fix libxo output being locale dependant

by being locale dependant the json export is invalid in locales where
the separator for float is a comma.
The Json and the XML are invalid for login-time when days contains
contains characters which are not unicode.

Forcing locale to be C, makes this json and xml output valid and also
identical accross locales, so reliable for parsers

PR: 276304
Reported by: Vedran Miletic <vedran@miletic.net>

(cherry picked from commit bd490be57438a82c22d1274bc58d51142b63f4a0)