Test the OCB-AES reference implementation

This tests cryptographic primitives implemented by others.  It uses the same
interfaces and indeed the same compiled object code as the Mosh client and
server.  It does not particularly test any code written for the Mosh project.

src/crypto/prng.h

@@ -72,6 +72,12 @@ class PRNG {
     fill( &x, 1 );
     return x;
   }
+
+  uint32_t uint32() {
+    uint32_t x;
+    fill( &x, 4 );
+    return x;
+  }
 };
 
 #endif