change RandomDevice(unlimited::Bool) to RandomDevice(; unlimited::Bool)

[rfourquet]

Making unlimited a keyword argument is mainly for 2 reasons:

1. it seems better that the argument(s) of an RNG constructor match those of srand (cf. srand docstring), i.e. that the positional argument(s) specify a seed.
2. it is available only on non-Windows systems, and is not a functionality that is obviously expected, so it feels more "right" to name it (i.e. what RandomDevice(false) does is not guessable).