Initialize the slots correctly when enabling ABS_MT_SLOT

Peter Hutterer requested to merge whot/libevdev:wip/slot-init into master

Previously, enabling or disabling ABS_MT_SLOT would not change the actual slots, it was treated as a normal bitflag. This means we couldn't initialize a libevdev context from scratch and have it behave like a correct MT context.

Fixes #4 (closed)

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