[v9_10] check for arc4random_addrandom()

3840.	[port]		Check for arc4random_addrandom() before using it;
			it's been removed from OpenBSD 5.5. [RT #35907]

(cherry picked from commit 1ea6e09c37)

lib/isc/random.c

@@ -67,8 +67,16 @@ isc_random_seed(isc_uint32_t seed)
 
 #ifndef HAVE_ARC4RANDOM
 	srand(seed);
-#else
+#elif defined(HAVE_ARC4RANDOM_ADDRANDOM)
 	arc4random_addrandom((u_char *) &seed, sizeof(isc_uint32_t));
+#else
+	/*
+	 * If arcrandom() is available and no corresponding seeding
+	 * function arc4random_addrandom() is available, no seeding is
+	 * done on such platforms (e.g., OpenBSD 5.5). This is because
+	 * the OS itself is supposed to seed the RNG and it is assumed
+	 * that no explicit seeding is required.
+	 */
 #endif
 }