Interface with libudev to get notified when a new joystick appears.
The code is in IH_Linux_Event rather than IH_Linux_Joystick because it
seemed like the former uses evdev (standard for most devices nowadays)
whereas the latter uses the old joydev API. The result should be the
same in any case...
Implementation is a bit messy due to LinuxInputManager only scanning for
devices from its constructor, this forces RageInput to have an exception
for LinuxInputManager. Possibly with more involved surgery a cleaner
solution could be devised...
Thomas Calvert