override native arc4random_buf on FreeBSD

The FreeBSD-native arc4random_buf implementation falls back to weak
sources of entropy if the sysctl fails. Remove these dangerous fallbacks
by overriding locally.

Unfortunately, pthread_atfork() is broken on FreeBSD (at least 9 and 10)
if a program does not link to -lthr. Callbacks registered with
pthread_atfork() simply fail silently. So, it is not always possible to
detect a PID wraparound. I wish we could do better.

This improves arc4random_buf's safety compared to the native FreeBSD
implementation. Tested on FreeBSD 9 and 10.

ok beck@ deraadt@

crypto/compat/arc4random.h

@@ -1,7 +1,12 @@
 #ifndef LIBCRYPTOCOMPAT_ARC4RANDOM_H
 #define LIBCRYPTOCOMPAT_ARC4RANDOM_H
 
-#if defined(__linux__)
+#include <sys/param.h>
+
+#if defined(__FreeBSD__)
+#include "arc4random_freebsd.h"
+
+#elif defined(__linux__)
 #include "arc4random_linux.h"
 
 #elif defined(__APPLE__)