Really use prefix JSON_C_OBJECT_ADD_*

Really use prefix JSON_C_OBJECT_ADD_*

This change introduces JSON_C_OBJECT_ADD_CONSTANT_KEY
as a replacement of JSON_C_OBJECT_KEY_IS_CONSTANT.

The description of json_object_object_add_ex tells to
look at the flags JSON_C_OBJECT_ADD_* but it is not
for JSON_C_OBJECT_KEY_IS_CONSTANT.

From the point of vue of a developper using json-c,
the function json_object_object_add_ex is mainly used,
not the hash facility, it seems more natural to provide
a regular naming of prefix JSON_C_OBJECT_ADD_CONSTANT_KEY.