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

0c49e52 build: remove unneeded getentropy detection (HAVE_GETENTROPY) (Sebastian Falbesoner)
5cd15ff random: use arc4random on OpenBSD (Sebastian Falbesoner)

Pull request description:

  Inspired by a discussion on obtaining randomness on various OSes in a secp256k1 PR (bitcoin-core/secp256k1#748 (comment), 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 0c49e52

Tree-SHA512: b5ed3d0718962c5a3839db9a28f93d08a0ac93094cc664f83bc4cf1cfad25049e6240b7b81fe06b71e6a3a0ca24a2c337eab088abec5470ad014e10c04fdb216

Author: laanwj

configure.ac

@@ -1112,13 +1112,6 @@ AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <unistd.h>
  [ AC_MSG_RESULT([no])]
 )
 
-AC_MSG_CHECKING([for getentropy])
-AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <unistd.h>]],
- [[ getentropy(nullptr, 32) ]])],
- [ AC_MSG_RESULT([yes]); AC_DEFINE([HAVE_GETENTROPY], [1], [Define this symbol if the BSD getentropy system call is available]) ],
- [ AC_MSG_RESULT([no])]
-)
-
 AC_MSG_CHECKING([for getentropy via random.h])
 AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <unistd.h>
  #include <sys/random.h>]],

src/random.cpp

@@ -32,10 +32,8 @@
 #include <sys/syscall.h>
 #include <linux/random.h>
 #endif
-#if defined(HAVE_GETENTROPY) || (defined(HAVE_GETENTROPY_RAND) && defined(MAC_OSX))
-#include <unistd.h>
-#endif
 #if defined(HAVE_GETENTROPY_RAND) && defined(MAC_OSX)
+#include <unistd.h>
 #include <sys/random.h>
 #endif
 #ifdef HAVE_SYSCTL_ARND
@@ -305,16 +303,14 @@ void GetOSRand(unsigned char *ent32)
             RandFailure();
         }
     }
-#elif defined(HAVE_GETENTROPY) && defined(__OpenBSD__)
-    /* On OpenBSD this can return up to 256 bytes of entropy, will return an
-     * error if more are requested.
-     * The call cannot return less than the requested number of bytes.
-       getentropy is explicitly limited to openbsd here, as a similar (but not
-       the same) function may exist on other platforms via glibc.
+#elif defined(__OpenBSD__)
+    /* OpenBSD. From the arc4random(3) man page:
+       "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."
+       The function call is always successful.
      */
-    if (getentropy(ent32, NUM_OS_RANDOM_BYTES) != 0) {
-        RandFailure();
-    }
+    arc4random_buf(ent32, NUM_OS_RANDOM_BYTES);
     // Silence a compiler warning about unused function.
     (void)GetDevURandom;
 #elif defined(HAVE_GETENTROPY_RAND) && defined(MAC_OSX)