json-c: Remove escaping of slashs

By default json-c escapes the slashes. This is almost ugly
and unuseful. But the resolution (with JSON_C_TO_STRING_NOSLASHESCAPE)
was introduced lately by json-c. Integrate it as much as possible.

Change-Id: Ia9c4bee78dc11df1ee9640cb04311991bd860e43
Signed-off-by: José Bollo <jose.bollo@iot.bzh>