Light wrapper around a file descriptor so it can be used safely with dbus.

Why do we need this? Because we want to safely pass a file descriptor around using the DBusMessage class.

Why not just use an integer? Because although it's obviously fine to pass an integer around, the life time of the file descriptor can get lost. This class uses dup(2) to ensure that if the object was created with a valid file descriptor in the first place then it and all copy constructed objects will have a valid file descriptor.