Stop using /dev/urandom, except on Linux, and then only when getrandom() isn't available

[briansmith]

Right now, the default implementation of ring::rand is to read from /dev/urandom. However, AFAICT, except for Redox, /dev/urandom either doesn't work, or there is something better that should be used instead. So, let's stop defaulting to /dev/urandom.

The main effect of this is that platforms that currently don't have an implementation of ring::rand will break. As far as I know, there are no ring users using these alternative operating systems, so we won't lose anything by removing the fallback.

Switching away from /dev/urandom on OpenBSD is issue #316.
Switching away from /dev/urandom on FreeBSD is issue #326.

[briansmith]

This was done in efb6ede. This will break FreeBSD, OpenBSD, and similar platforms unless/until PRs are merged that fix them with an implementation similar to the Linux getrandom()-based implementation.