Avoid magic "0" argument to prepareForShutdown().

Backported from Disque.

src/server.c

@@ -1125,7 +1125,7 @@ int serverCron(struct aeEventLoop *eventLoop, long long id, void *clientData) {
     /* We received a SIGTERM, shutting down here in a safe way, as it is
      * not ok doing so inside the signal handler. */
     if (server.shutdown_asap) {
-        if (prepareForShutdown(0) == C_OK) exit(0);
+        if (prepareForShutdown(SHUTDOWN_NOFLAGS) == C_OK) exit(0);
         serverLog(LL_WARNING,"SIGTERM received but errors trying to shut down the server, check the logs for more information");
         server.shutdown_asap = 0;
     }

src/server.h

@@ -368,9 +368,10 @@ typedef long long mstime_t; /* millisecond time type. */
 #define UNIT_MILLISECONDS 1
 
 /* SHUTDOWN flags */
-#define SHUTDOWN_SAVE 1       /* Force SAVE on SHUTDOWN even if no save
-                                       points are configured. */
-#define SHUTDOWN_NOSAVE 2     /* Don't SAVE on SHUTDOWN. */
+#define SHUTDOWN_NOFLAGS 0      /* No flags. */
+#define SHUTDOWN_SAVE 1         /* Force SAVE on SHUTDOWN even if no save
+                                   points are configured. */
+#define SHUTDOWN_NOSAVE 2       /* Don't SAVE on SHUTDOWN. */
 
 /* Command call flags, see call() function */
 #define CMD_CALL_NONE 0