add GetRandBytes() as wrapper for RAND_bytes()

- add a small wrapper in util around RAND_bytes() and replace with
  GetRandBytes() in the code to log errors from calling RAND_bytes()
- remove OpenSSL header rand.h where no longer needed

src/util.h

@@ -103,6 +103,7 @@ extern bool fLogTimestamps;
 extern bool fLogIPs;
 extern volatile bool fReopenDebugLog;
 
+bool GetRandBytes(unsigned char *buf, int num);
 void RandAddSeed();
 void RandAddSeedPerfmon();
 void SetupEnvironment();