MINOR: proxy: store the default target into the frontend's configuration

Some services such as peers and CLI pre-set the target applet immediately
during accept(), and for this reason they're forced to have a dedicated
accept() function which does not even properly follow everything the regular
one does (eg: sndbuf/rcvbuf/linger/nodelay are not set, etc).

Let's store the default target when known into the frontend's config so that
it's session_accept() which automatically sets it.
diff --git a/include/types/proxy.h b/include/types/proxy.h
index 30409f9..0cd00ee 100644
--- a/include/types/proxy.h
+++ b/include/types/proxy.h
@@ -329,6 +329,7 @@
 	time_t last_change;			/* last time, when the state was changed */
 	int (*accept)(struct session *s);       /* application layer's accept() */
 	struct conn_src conn_src;               /* connection source settings */
+	enum obj_type *default_target;		/* default target to use for accepted sessions or NULL */
 	struct proxy *next;
 
 	unsigned int log_count;			/* number of logs produced by the frontend */