Merge bitcoin/bitcoin#24238: random: use arc4random on OpenBSD

0c49e52b22be1baa8d51670e4f3c437fd3c0baa7 build: remove unneeded getentropy detection (HAVE_GETENTROPY) (Sebastian Falbesoner)
5cd15ffdceace3a077d4719ef7c1704336d602e1 random: use arc4random on OpenBSD (Sebastian Falbesoner)

Pull request description:

  Inspired by a discussion on obtaining randomness on various OSes in a secp256k1 PR (https://github.com/bitcoin-core/secp256k1/pull/748#discussion_r524605472, see also https://bitcoincore.reviews/libsecp256k1-748), I think it makes sense to follow best practices and use `arc4random_buf` rather than `getentropy` on OpenBSD in our random module.

  The [getentropy(2) man page](https://man.openbsd.org/getentropy.2) states:
  ```
  getentropy() is not intended for regular code; please use the
  arc4random(3) family of functions instead.
  ```

  The [arc4random(3) man page](https://man.openbsd.org/arc4random.3) states:

  ```
  Use of these functions is encouraged for almost all random number
  consumption because the other interfaces are deficient in either quality,
  portability, standardization, or availability.
  ```
  On the linked PR discussion worries about using RC4 internally has been expressed (see https://security.stackexchange.com/questions/85601/is-arc4random-secure-enough/172905#172905), but this would only affect users of OpenBSD <5.5, using a version that was released more than 8 years ago.

ACKs for top commit:
  laanwj:
    Tested ACK 0c49e52b22be1baa8d51670e4f3c437fd3c0baa7

Tree-SHA512: b5ed3d0718962c5a3839db9a28f93d08a0ac93094cc664f83bc4cf1cfad25049e6240b7b81fe06b71e6a3a0ca24a2c337eab088abec5470ad014e10c04fdb216

0 files changed, 0 insertions, 0 deletions