Change method by which port errors are signalled. Previously a

Change method by which port errors are signalled.  Previously a
condition handler was bound for every I/O operation, which made I/O
painfully slow as each binding cost at least one hundred machine
instructions.  Now, an I/O channel can have an associated port, and
when a system-call error occurs, the file descriptor associated with
the error is mapped back to the associated port, if any, and a port
error is signalled instead.