These are available in sandboxes without access to files or
devices. Also [they are safer and more straightforward](https://en.wikipedia.org/wiki/Entropy-supplying_system_calls)
to use than `/dev/urandom` as reading from a file has quite a few edge
cases:

- Linux: `getrandom(buf, buflen, 0)`. [getrandom(2)](http://man7.org/linux/man-pages/man2/getrandom.2.html)
  was introduced in version 3.17 of the Linux kernel.
- OpenBSD: `getentropy(buf, buflen)`. The [getentropy(2)](http://man.openbsd.org/cgi-bin/man.cgi/OpenBSD-current/man2/getentropy.2)
  function appeared in OpenBSD 5.6.
- FreeBSD and NetBSD: `sysctl(KERN_ARND)`. Not sure when this was added
  but it has existed for quite a while.

Alternatives:

- Linux has sysctl `CTL_KERN` / `KERN_RANDOM` / `RANDOM_UUID`
  which gives 16 bytes of randomness. This may be available
  on older kernels, however [sysctl is deprecated on Linux](https://lwn.net/Articles/605392/)
  and even removed in some distros so we shouldn't use it.

Add tests for `GetOSRand()`:

- Test that no error happens (otherwise `RandFailure()` which aborts)
- Test that all 32 bytes are overwritten (initialize with zeros, try multiple times)

Discussion:

- When to use these? Currently they are always used when available.
  Another option would be to use them only when `/dev/urandom` is not
  available. But this would mean these code paths receive less testing,
  and I'm not sure there is any reason to prefer `/dev/urandom`.

Closes: #9676

Move the OS random test to a sanity check function that is called every
time bitcoind is initialized.

Keep `src/test/random_tests.cpp` for the case that later random tests
are added, and keep a rudimentary test that just calls the sanity check.

If the code was compiled with newer (>=3.17) kernel headers but executed
on a system without the system call, every use of random would crash the
program. Add a fallback for that case.

We'll need a version of SipHash for tuples of 256 bits and 32 bits
data, when CCoinsViewCache switches from using txids to COutPoints as
keys.

This is a move-only commit with the exception of changes to includes.

Cherry-picked from: 4fb789e