Use the new ChaCha20 CSPRNG on Linux

[ahomescu]

Now that we have a CSPRNG in the selfrando source tree, we could also use it on Linux. We currently read all our random numbers directly from /dev/urandom, but instead could just use that to seed our RNG.

[Maryse47]

Isn't using kernel CSPRNG always recommended than userspace one? Nowadays you can just use getrandom syscall instead of /dev/urandom if you wish.