[srv.init] Load initial programs in srv.init

Add a simple ELF loader to srv.init to load and start any module_program
parameters passed from the bootloader. Also creates stacks for newly
created threads.

Also update thread creation in testapp to create stacks.

src/user/srv.init/loader.h

@@ -0,0 +1,10 @@
+#pragma once
+/// \file loader.h
+/// Routines for loading and starting other programs
+
+namespace kernel {
+namespace init {
+    struct module_program;
+}}
+
+bool load_program(const kernel::init::module_program &prog, char *err_msg);