MINOR: applet: add an execution timeout

This patch only declares the execution timeout variable. The applet
must be respect or ignore it. For example an applet who doing some
network accesses and doesn't control its execution yime should use
this timeout.

include/types/applet.h

@@ -40,6 +40,7 @@ struct applet {
 	                                     expect 1 if ok, 0 if an error occurs, -1 if miss data. */
 	void (*fct)(struct appctx *);      /* internal I/O handler, may never be NULL */
 	void (*release)(struct appctx *);  /* callback to release resources, may be NULL */
+	unsigned int timeout;              /* execution timeout. */
 };
 
 /* Context of a running applet. */