JSON: Added json_object_add_member() function

This commit increases code reuse by adding the json_object_add_member()
function whose functionality was formerly repeated numerous times in
jsonutils.c.
JSON CGIs: Removed 8 kB string size limitation

There were two places in jsonutils.c where a static 8 kB buffer was
allocated for string processing. This buffer is now dynamically
allocated and string sizes are only limited by the amount of RAM
on a system.