add seed_rng namespace and files

In order to separate the "seed source" objects from the actual
PRG objects, this adds a new file and name space "seed_rng" which
can be used by rng's which are constructed without a seed.

The implementation has preprocessor guards, and should be either
boost::random_device or just the system time if that is
unavailable.

Since it goes on server as well this file also needs to be in
wesnoth-core.