The io library keeps track of an optional timeout for each descriptor. The
timeout is a specific moment in time, stored in a tai6464 variable.
io_timeout(d,t) sets the timeout for descriptor d to t.
io_timeout has no return value; it always succeeds. (Space to store the timeout
was already allocated as part of the descriptor.) It has no effect if d is not
the number of a descriptor.
Note that you have to call io_timeouted to be notified of timeouts, just
like you are calling io_canread or io_canwrite for read and write