Sentinel: Reject config from STDIN

Sentinel needs to die with a more accurate error message
when attempted to open a config from STDIN.

See: https://groups.google.com/forum/#!topic/redis-db/sYx7VNMWaNM

src/redis.c

@@ -3275,6 +3275,13 @@ int main(int argc, char **argv) {
             }
             j++;
         }
+        if (server.sentinel_mode && configfile && *configfile == '-') {
+            redisLog(REDIS_WARNING,
+                "Sentinel config from STDIN not allowed.");
+            redisLog(REDIS_WARNING,
+                "Sentinel needs config file on disk to save state.  Exiting...");
+            exit(1);
+        }
         if (configfile) server.configfile = getAbsolutePath(configfile);
         resetServerSaveParams();
         loadServerConfig(configfile,options);