random: use arc4random on OpenBSD

Following best practices on OpenBSD. The getentropy(2) man page states:
"getentropy() is not intended for regular code;
 please use the arc4random(3) family of functions instead."

Author: theStack

src/random.cpp

@@ -305,16 +305,14 @@ 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)