[kernel] Split loading from scheduler

In preparation for moving things to the init process, move process
loading out of the scheduler. memory_bootstrap now has a
load_simple_process function for mapping an args::program into memory,
and the stack setup has been simplified (though all the initv values are
still being added by the kernel - this needs rework) and normalized to
use the thread::add_thunk_user code path.

src/kernel/objects/process.cpp

@@ -106,7 +106,9 @@ process::create_thread(uint8_t priority, bool user)
 				vm_flags::zero|vm_flags::write);
 		m_space.add(stack_top - stack_size, vma);
 
-		th->tcb()->rsp3 = stack_top;
+		// Space for null frame - because the page gets zeroed on
+		// allocation, just pointing rsp here does the trick
+		th->tcb()->rsp3 = stack_top - 2 * sizeof(uint64_t);
 	}
 
 	m_threads.append(th);