Networking layer speedup and refactoring in --net-only mode.

The networking layer is still a toy however now there is not a blocking
sleep when we need to wait to process for new clients, but a basic form
of multiplexing is used to signal when at least one client needs some
care.

The code was also partially refactored, but more work is needed to make
it more straightforward.
Free the client on best effort write(2) short write.
In debug mode log when accept fails.