This is a simple example of reading/writing messages back and forth.

The program assumes that it starts up with a handle value to a channel and
that something on the other end of that channel is sending some number of
Echo calls along the pipe. The program:
*) waits for the channel to be readable
*) reads an incoming message
*) validates that it seems sane
*) formats a reply message
*) sends the reply
*) goes back to waiting
until the channel peer is closed or any other error is encountered.