MXS-1220: Add /users/ resource

The /users/ resource shows all user accounts that can be used with
MaxScale. This resource is further split into two resource collections,
/users/inet and /users/unix, which contain the network users and Linux
accounts respectively.

include/maxscale/json_api.h

@@ -32,6 +32,7 @@ MXS_BEGIN_DECLS
 #define MXS_JSON_API_LOGS      "/maxscale/logs/"
 #define MXS_JSON_API_TASKS     "/maxscale/tasks/"
 #define MXS_JSON_API_MODULES   "/maxscale/modules/"
+#define MXS_JSON_API_USERS     "/users/"
 
 /**
  * @brief Create a JSON object