[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/thread.h

@@ -50,6 +50,8 @@ public:
 	/// Destructor
 	virtual ~thread();
 
+	static constexpr kobject::type type = kobject::type::thread;
+
 	/// Get the currently executing thread.
 	static thread & current();