[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/endpoint.h

@@ -13,6 +13,8 @@ public:
 	endpoint();
 	virtual ~endpoint();
 
+	static constexpr kobject::type type = kobject::type::endpoint;
+
 	/// Close the endpoint, waking all waiting processes with an error
 	void close();