[kernel] Give processes and threads self handles

It was not consistent how processes got handles to themselves or their
threads, ending up with double entries. Now make such handles automatic
and expose them with new self_handle() methods.

src/kernel/objects/process.cpp

@@ -21,13 +21,16 @@ process::process() :
 	m_state {state::running}
 {
 	s_processes.append(this);
+
+	j6_handle_t self = add_handle(this);
+	kassert(self == self_handle(), "Process self-handle is not 0");
 }
 
 // The "kernel process"-only constructor
 process::process(page_table *kpml4) :
 	kobject {kobject::type::process},
 	m_space {kpml4},
-	m_next_handle {0},
+	m_next_handle {self_handle()+1},
 	m_state {state::running}
 {
 }
@@ -120,6 +123,7 @@ process::thread_exited(thread *th)
 	kassert(&th->m_parent == this, "Process got thread_exited for non-child!");
 	uint32_t status = th->m_return_code;
 	m_threads.remove_swap(th);
+	remove_handle(th->self_handle());
 	delete th;
 
 	if (m_threads.count() == 0) {