Add support for uinput device creation

Add support for uinput device creation

This lets libevdev provide a relatively generic interface for the
creation of uinput devices so we don't need to duplicate this across
multiple projects.

Most of this is lifted from the current test implementation, with a
couple of minor changes.

EV_REP needs special handling:
   Kernel allows to set the EV_REP bit, it doesn't set REP_* bits (which we
   wrap anyway) but it will also set the default values (500, 33).

Device node is guessed based on the sysfs path:
   The sysfs path contains a eventN file, that corresponds to our
   /dev/input/eventN number. Use it so clients can quickly get the device
   node, without a libudev dependency.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>