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/main.cpp

@@ -4370,7 +4370,7 @@ bool SendMessages(CNode* pto, bool fSendTrickle)
         if (pingSend) {
             uint64_t nonce = 0;
             while (nonce == 0) {
-                RAND_bytes((unsigned char*)&nonce, sizeof(nonce));
+                GetRandBytes((unsigned char*)&nonce, sizeof(nonce));
             }
             pto->fPingQueued = false;
             pto->nPingUsecStart = GetTimeMicros();