[kernel] Add endpoint object and related syscalls

The endpoint object adds synchronous IPC. Also added the wait-type of
'object' to threads.

src/kernel/objects/endpoint.h

@@ -0,0 +1,53 @@
+#pragma once
+/// \file endpoint.h
+/// Definition of endpoint kobject types
+
+#include "j6/signals.h"
+#include "objects/kobject.h"
+
+/// Endpoints are objects that enable synchronous message-passing IPC
+class endpoint :
+	public kobject
+{
+public:
+	endpoint();
+	virtual ~endpoint();
+
+	/// Close the endpoint, waking all waiting processes with an error
+	void close();
+
+	/// Check if the endpoint has space for a message to be sent
+	inline bool can_send() const { return check_signal(j6_signal_endpoint_can_send); }
+
+	/// Check if the endpoint has a message wiating already
+	inline bool can_receive() const { return check_signal(j6_signal_endpoint_can_recv); }
+
+	/// Send a message to a thread waiting to receive on this endpoint. If no threads
+	/// are currently trying to receive, block the current thread.
+	/// \arg len   The size in bytes of the message
+	/// \arg data  The message data
+	/// \returns   j6_status_ok on success
+	j6_status_t send(size_t len, void *data);
+
+	/// Receive a message from a thread waiting to send on this endpoint. If no threads
+	/// are currently trying to send, block the current thread.
+	/// \arg len   [in] The size in bytes of the buffer [out] Number of bytes in the message
+	/// \arg data  Buffer for copying message data into
+	/// \returns   j6_status_ok on success
+	j6_status_t receive(size_t *len, void *data);
+
+private:
+	struct thread_data
+	{
+		thread *th;
+		void *data;
+		union {
+			size_t *len_p;
+			size_t len;
+		};
+	};
+
+	j6_status_t do_message_copy(const thread_data &sender, thread_data &receiver);
+
+	kutil::vector<thread_data> m_blocked;
+};