ktls_test: Permit connecting to a remote echo server for tests.

Previously ktls tests always executed over a local socket pair.
ktls.host can be set to a host to connect to with a single socket
instead. The remote end is expected to echo back any data received
(such as the echo service). The port can be set with ktls.port which
defaults to "echo".

This is primarily useful to permit testing NIC TLS offload use cases
where the traffic needs to transit the NIC.

Note that the variables must be set via
'kyua -v test_suites.FreeBSD.ktls.host=host'.

Reviewed by: markj
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D35426