Rescan interfaces before dropping privileges

The ns_interfacemgr_scan() now requires the loopmgr to be running, so we need to end exclusive mode for the rescan and then begin it again.

This is relatively safe operation (because the scan happens on the timer anyway), but we need to ensure that we won't load the configuration from different threads. This is already the case because the initial load happens on the main thread and the control channel also listens just on the main loop.

Closes #3583 (closed)