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.

server/core/gateway.cc

@@ -36,6 +36,7 @@
 #include <sys/wait.h>
 
 #include <maxscale/alloc.h>
+#include <maxscale/adminusers.h>
 #include <maxscale/dcb.h>
 #include <maxscale/housekeeper.h>
 #include <maxscale/log_manager.h>
@@ -1882,6 +1883,9 @@ int main(int argc, char **argv)
 
     dcb_global_init();
 
+    /** Load the admin users */
+    admin_users_init();
+
     /* Initialize the internal query classifier. The plugin will be initialized
      * via the module initialization below.
      */