tests: Add missing include for writev/readv.
tests: Add test_echo_tcp_connect test.

This tests some corner cases we fixed in the past.
tests: Add test_echo_tcp_socket_options().
torture: Generate network traces for each test.
torture: Add TORTURE_SKIP_CLEANUP to not delete socket_dir.
tests: Remove unused variable warning

The variables were unused if SIOCGPGRP wasn't defined.
tests: writev/readv must have iovec > 0

writev/readv with iovec == 0 works on Linux, but that is not portable.