Sentinel: test for writable config file.

This commit introduces a funciton called when Sentinel is ready for
normal operations to avoid putting Sentinel specific stuff in redis.c.

src/redis.c

@@ -3012,7 +3012,7 @@ int main(int argc, char **argv) {
         if (server.sofd > 0)
             redisLog(REDIS_NOTICE,"The server is now ready to accept connections at %s", server.unixsocket);
     } else {
-        redisLog(REDIS_WARNING,"Sentinel runid is %s", server.runid);
+        sentinelIsRunning();
     }
 
     /* Warning the user about suspicious maxmemory setting. */