Initialize joystick on plug after module loaded

When using "module joystick" plugging joystick out and in again doesn't affect it's work. But if joystick module loaded when no joysticks were available then it will not be working when plugged. Joystick module should check joystick availability or react on device changes.

If mavproxy autostarted on embedded computer in GCS then start without joystick will deny it's work after joystick plugged in.

Start by tracing the joystick module's initialization path and how it discovers available devices when MAVProxy starts. Reproduce the case where no joystick is present at module load, then verify that plugging one in enables joystick operation without restarting MAVProxy.

Written by the indexing model from the issue text.