Change implementation of readahead in input channels that require it.

Change implementation of readahead in input channels that require it.
Readahead is now buffered in the input queue by means of a new
operation to push a message back onto the head of the queue.  Also,
there is now a mechanism to delay starting the readahead thread until
the first read occurs on the channel -- this is used for input pipes
that are passed to child processes and never read from the Scheme end.
It's important that such pipes not be read from.