alarm(3) cannot fail, remove error handling.
suggested by millert@

Connect netcat client to server and close stdin. Then netcat waits
for the other side to terminate. Check that netcat is sleeping and
does not run in a busy loop. Add tests for TCP, TLS, and UDP.

Test the netcat -N feature. The test program can send and receive
messages, shutdown write, and wait for EOF in arbitray order. Since
revision 1.211 of netcat.c some of the cases fail.

Implement custom TCP client and server to test netcat corner cases.