Put out a warning if xorg.conf has InputDevice sections, but these aren't
referenced in the used ServerLayout. This is only performed if AllowEmptyInput
is enabled.
The reason behind this is that the server used to auto-add the first
mouse/keyboard sections if none where referenced. Now, with HAL and AEI
enabled by default, setups that relied on this auto-adding break and are left
without input devices. The least we can do is warn them.
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