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/rpcserver.cpp

@@ -531,7 +531,7 @@ void StartRPCThreads()
          (mapArgs["-rpcuser"] == mapArgs["-rpcpassword"])) && Params().RequireRPCPassword())
     {
         unsigned char rand_pwd[32];
-        RAND_bytes(rand_pwd, 32);
+        GetRandBytes(rand_pwd, 32);
         string strWhatAmI = "To use bitcoind";
         if (mapArgs.count("-server"))
             strWhatAmI = strprintf(_("To use the %s option"), "\"-server\"");