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/src/session.c b/src/session.c
index 94fc875..7a00b0d 100644
--- a/src/session.c
+++ b/src/session.c
@@ -481,7 +481,7 @@
 		s->si[1].flags |= SI_FL_INDEP_STR;
 
 	session_init_srv_conn(s);
-	s->target = NULL;
+	s->target = p->default_target; /* used by peers and CLI */
 	s->pend_pos = NULL;
 
 	/* init store persistence */