class copier :copy_task -> Unixqueue.event_system ->[unit]engine
copy_task argument.
The task is done when all input descriptors are at EOF. See
the description of copy_task for details, especially whether
the descriptors are closed or not.
On error or abort, the descriptors are only closed if they had been closed on regular EOF.
The semantics of the engine is undefined if one of the descriptors is not stream-oriented.
TODO: This class cannot yet cope with Win32 named piped.
