Wait for uinput device creation

[georgefst]

Unfortunately the principled approach in the first commit isn't enough without the arbitrary wait in the second, which possibly makes the first redundant. This can be seen with echo 'testing' | $(cabal list-bin uinput-echo).

Note that on device creation we see two events in the registerFd callback. In these, (UDev.getDevnode udevDev, UDev.getSyspath udevDev) gives us (ignoring IO and errors) (Nothing, deviceSyspath uinputDev) then (Just deviceDevnode, deviceSyspath uinputDev </> baseName (deviceDevnode uinputDev)). We look for the second event, so that we know (in theory...) that initialisation has finished, and check the first element because it's simpler, though this may not be the best approach if deviceSyspath uinputDev is Nothing, e.g. on FreeBSD.