[kernel] Add syscall helpers

Added the syscalls/helpers.h file to templatize common kobject syscall
operations. Also moved most syscall implementations to using
process::current() and thread::current() instead of asking the
scheduler.

src/kernel/objects/channel.h

@@ -14,6 +14,8 @@ public:
 	channel();
 	virtual ~channel();
 
+	static constexpr kobject::type type = kobject::type::channel;
+
 	/// Check if the channel has space for a message to be sent
 	inline bool can_send() const { return check_signal(j6_signal_channel_can_send); }