CLEANUP: conn-stream: rename cs_register_applet() to cs_applet_create()

cs_register_applet() was not a good name because it suggests it happens
during startup, just like any other registration mechanisms..
diff --git a/src/backend.c b/src/backend.c
index ec45c71..8255ffb 100644
--- a/src/backend.c
+++ b/src/backend.c
@@ -2130,7 +2130,7 @@
 		struct appctx *appctx = cs_appctx(s->csb);
 
 		if (!appctx || appctx->applet != __objt_applet(s->target))
-			appctx = cs_register_applet(cs, objt_applet(s->target));
+			appctx = cs_applet_create(cs, objt_applet(s->target));
 
 		if (!appctx) {
 			/* No more memory, let's immediately abort. Force the