[kernel] Consolodate koid and close syscalls

A number of object types had _close or _koid syscalls. Moved those to be
generic for kobject.

src/kernel/objects/kobject.cpp

@@ -61,6 +61,12 @@ kobject::notify_signal_observers()
 	}
 }
 
+void
+kobject::close()
+{
+	assert_signal(j6_signal_closed);
+}
+
 void
 kobject::on_no_handles()
 {