util: Specific GetOSRandom for Linux/FreeBSD/OpenBSD

These are available in sandboxes without access to files or
devices. Also [they are safer and more straightforward](https://en.wikipedia.org/wiki/Entropy-supplying_system_calls)
to use than `/dev/urandom` as reading from a file has quite a few edge
cases:

- Linux: `getrandom(buf, buflen, 0)`. [getrandom(2)](http://man7.org/linux/man-pages/man2/getrandom.2.html)
  was introduced in version 3.17 of the Linux kernel.
- OpenBSD: `getentropy(buf, buflen)`. The [getentropy(2)](http://man.openbsd.org/cgi-bin/man.cgi/OpenBSD-current/man2/getentropy.2)
  function appeared in OpenBSD 5.6.
- FreeBSD and NetBSD: `sysctl(KERN_ARND)`. Not sure when this was added
  but it has existed for quite a while.

Alternatives:

- Linux has sysctl `CTL_KERN` / `KERN_RANDOM` / `RANDOM_UUID`
  which gives 16 bytes of randomness. This may be available
  on older kernels, however [sysctl is deprecated on Linux](https://lwn.net/Articles/605392/)
  and even removed in some distros so we shouldn't use it.

Add tests for `GetOSRand()`:

- Test that no error happens (otherwise `RandFailure()` which aborts)
- Test that all 32 bytes are overwritten (initialize with zeros, try multiple times)

Discussion:

- When to use these? Currently they are always used when available.
  Another option would be to use them only when `/dev/urandom` is not
  available. But this would mean these code paths receive less testing,
  and I'm not sure there is any reason to prefer `/dev/urandom`.

Closes: #9676

configure.ac

@@ -558,6 +558,33 @@ AC_LINK_IFELSE([AC_LANG_SOURCE([
   ]
 )
 
+# Check for different ways of gathering OS randomness
+AC_MSG_CHECKING(for Linux getrandom syscall)
+AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <unistd.h>
+  #include <sys/syscall.h>
+  #include <linux/random.h>]],
+ [[ syscall(SYS_getrandom, nullptr, 32, 0); ]])],
+ [ AC_MSG_RESULT(yes); AC_DEFINE(HAVE_SYS_GETRANDOM, 1,[Define this symbol if the Linux getrandom system call is available]) ],
+ [ 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 sysctl KERN_ARND)
+AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <sys/types.h>
+  #include <sys/sysctl.h>]],
+ [[ static const int name[2] = {CTL_KERN, KERN_ARND};
+    sysctl(name, 2, nullptr, nullptr, nullptr, 0); ]])],
+ [ AC_MSG_RESULT(yes); AC_DEFINE(HAVE_SYSCTL_ARND, 1,[Define this symbol if the BSD sysctl(KERN_ARND) is available]) ],
+ [ AC_MSG_RESULT(no)]
+)
+
+# Check for reduced exports
 if test x$use_reduce_exports = xyes; then
   AX_CHECK_COMPILE_FLAG([-fvisibility=hidden],[RE_CXXFLAGS="-fvisibility=hidden"],
   [AC_MSG_ERROR([Cannot set default symbol visibility. Use --disable-reduce-exports.])])