OpenBSD: Suggest using userspace PRNG where appropriate.

Specifically, following discussions with knowledgeable people, arc4random on OpenBSD is a userspace per-process ChaCha20 seeded PRNG and probably better used for larger amounts of randomness than getentropy. Therefore, suggest it in preference to getentropy, but leave getentropy for its suggested use i.e. seeding PRNGs. Resolves issue #17. Signed-off-by: Antony Vennard <antony@vennard.ch>

Author: diagprov

README.md

@@ -594,7 +594,7 @@ For cryptographic applications,
 Minimize the need for randomness through design and choice of primitives (for example [Ed25519](http://ed25519.cr.yp.to/) produces signatures deterministically).
 
 On Linux, use the [`getrandom()`](http://man7.org/linux/man-pages/man2/getrandom.2.html) system call, which ensures that the underlying PRNG has a high enough level entropy but will not "block" afterwards.
-On OpenBSD, use [`getentropy()`](https://man.openbsd.org/getentropy.2), which has a similar behavior and predates Linux' syscall.
+On OpenBSD, uze ['arc4random()'](https://man.openbsd.org/arc4random.3), which is a ChaCha20-based PRNG that calls `getentropy()` for its initial seed. Use this for large amounts of cryptographically-secure randomness. [`getentropy()`](https://man.openbsd.org/getentropy.2) has a 256-byte limit per call and is suitable for scenarios such as seeding PRNGs.
 On FreeBSD 12 and newer, both [`getrandom()`](https://www.freebsd.org/cgi/man.cgi?getrandom) and [`getentropy()`](https://www.freebsd.org/cgi/man.cgi?query=getentropy&sektion=3&apropos=0&manpath=FreeBSD+12.0-RELEASE+and+Ports) are available. Older versions only have the `KERN_ARND` `sysctl`.
 
 The OpenSSL API offers [`RAND_bytes()`](https://www.openssl.org/docs/man1.0.2/man3/RAND_bytes.html), which behaves differently depending on the platform and attempts to use reliable source of entropy when available. For example, on a Unix platform it would use `/dev/urandom` and the RDRAND/RDSEED instructions, if available, among others.