[CLEANUP] stream_interface: use inline functions to manipulate targets

The connection target involves a type and a union of pointers, let's
make the code cleaner using simple wrappers.
diff --git a/src/peers.c b/src/peers.c
index 06ad244..3e07a27 100644
--- a/src/peers.c
+++ b/src/peers.c
@@ -1157,8 +1157,7 @@
 	s->si[0].err_type = SI_ET_NONE;
 	s->si[0].err_loc = NULL;
 	s->si[0].connect   = NULL;
-	s->si[0].target.ptr.v = NULL;
-	s->si[0].target.type = TARG_TYPE_NONE;
+	clear_target(&s->si[0].target);
 	s->si[0].exp = TICK_ETERNITY;
 	s->si[0].flags = SI_FL_NONE;
 	if (s->fe->options2 & PR_O2_INDEPSTR)
@@ -1176,8 +1175,7 @@
 	s->si[1].err_type = SI_ET_NONE;
 	s->si[1].err_loc = NULL;
 	s->si[1].connect = tcpv4_connect_server;
-	s->si[1].target.ptr.p = s->be;
-	s->si[1].target.type = TARG_TYPE_PROXY;
+	set_target_proxy(&s->si[1].target, s->be);
 	s->si[1].exp = TICK_ETERNITY;
 	s->si[1].flags = SI_FL_NONE;
 	if (s->be->options2 & PR_O2_INDEPSTR)
@@ -1187,8 +1185,7 @@
 	s->si[1].release = NULL;
 
 	s->srv = s->srv_conn = NULL;
-	s->target.type = TARG_TYPE_NONE;
-	s->target.ptr.v = NULL;
+	clear_target(&s->target);
 	s->pend_pos = NULL;
 
 	/* init store persistence */