add src-netbsd branch appstack-src as a submodule

diff --git a/rumpsrc b/rumpsrc
new file mode 160000 (submodule)
index 0000000..9c03f22
--- /dev/null
+++ b/rumpsrc
@@ -0,0 +1 @@
+Subproject commit 9c03f22ad49f397480753f363dc0a93152c8a363