[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/syscalls/helpers.h

@@ -0,0 +1,40 @@
+#pragma once
+/// \file syscalls/helpers.h
+/// Utility functions for use in syscall handler implementations
+
+#include "j6/types.h"
+#include "objects/process.h"
+
+namespace syscalls {
+
+template <typename T, typename... Args>
+T * construct_handle(j6_handle_t *handle, Args... args)
+{
+	process &p = process::current();
+	T *o = new T {args...};
+	*handle = p.add_handle(o);
+	return o;
+}
+
+template <typename T>
+T * get_handle(j6_handle_t handle)
+{
+	process &p = process::current();
+	kobject *o = p.lookup_handle(handle);
+	if (!o || o->get_type() != T::type)
+		return nullptr;
+	return static_cast<T*>(o);
+}
+
+template <typename T>
+T * remove_handle(j6_handle_t handle)
+{
+	T *o = get_handle<T>(handle);
+	if (o) {
+		process &p = process::current();
+		p.remove_handle(handle);
+	}
+	return o;
+}
+
+}