Fix MULTI / EXEC rendering in MONITOR output.

Before of this commit it used to be like this:

MULTI
EXEC
... actual commands of the transaction ...

Because after all that is the natural order of things. Transaction
commands are queued and executed *only after* EXEC is called.

However this makes debugging with MONITOR a mess, so the code was
modified to provide a coherent output.

What happens is that MULTI is rendered in the MONITOR output as far as
possible, instead EXEC is propagated only after the transaction is
executed, or even in the case it fails because of WATCH, so in this case
you'll simply see:

MULTI
EXEC

An empty transaction.

src/redis.h

@@ -86,6 +86,7 @@
 #define REDIS_CMD_SORT_FOR_SCRIPT 256       /* "S" flag */
 #define REDIS_CMD_LOADING 512               /* "l" flag */
 #define REDIS_CMD_STALE 1024                /* "t" flag */
+#define REDIS_CMD_SKIP_MONITOR 2048         /* "M" flag */
 
 /* Object types */
 #define REDIS_STRING 0