[kernel] Add thead kobject class

Add the thread kernel API object and move the scheduler to use threads
instead of processes for scheduling and task switching.

src/kernel/objects/thread.cpp

@@ -0,0 +1,83 @@
+#include "j6/signals.h"
+#include "objects/thread.h"
+#include "scheduler.h"
+
+static constexpr j6_signal_t thread_default_signals = 0;
+
+thread::thread(page_table *pml4, priority_t pri) :
+	kobject(kobject::type::thread, thread_default_signals),
+	m_state(state::loading),
+	m_wait_type(wait_type::none),
+	m_wait_data(0),
+	m_wait_obj(0)
+{
+	TCB *tcbp = tcb();
+	tcbp->pml4 = pml4;
+	tcbp->priority = pri;
+	tcbp->thread_data = this;
+	set_state(state::ready);
+}
+
+void
+thread::wait_on_signals(kobject *obj, j6_signal_t signals)
+{
+	m_wait_type = wait_type::signal;
+	m_wait_data = signals;
+	clear_state(state::ready);
+}
+
+void
+thread::wait_on_time(uint64_t t)
+{
+	m_wait_type = wait_type::time;
+	m_wait_data = t;
+	clear_state(state::ready);
+}
+
+bool
+thread::wake_on_signals(kobject *obj, j6_signal_t signals)
+{
+	if (m_wait_type != wait_type::signal ||
+		(signals & m_wait_data) == 0)
+		return false;
+
+	m_wait_type = wait_type::none;
+	m_wait_data = j6_status_ok;
+	m_wait_obj = obj->koid();
+	set_state(state::ready);
+	return true;
+}
+
+bool
+thread::wake_on_time(uint64_t now)
+{
+	if (m_wait_type != wait_type::time ||
+		now < m_wait_data)
+		return false;
+
+	m_wait_type = wait_type::none;
+	m_wait_data = j6_status_ok;
+	m_wait_obj = 0;
+	set_state(state::ready);
+	return true;
+}
+
+void
+thread::wake_on_result(kobject *obj, j6_status_t result)
+{
+	m_wait_type = wait_type::none;
+	m_wait_data = result;
+	m_wait_obj = obj->koid();
+	set_state(state::ready);
+}
+
+void
+thread::exit(uint32_t code)
+{
+	// TODO: check if the process containing this thread
+	// needs to exit and clean up.
+	m_return_code = code;
+	set_state(state::exited);
+	clear_state(state::ready);
+}
+