Add zerosmtp-check to Networking

[msgwing]

Adds zerosmtp-check to Networking.

It answers one question: does outbound SMTP work from this machine? It tries the
ports, checks TLS, attempts authentication, and then explains the refusal the
server actually returned — including the ones clients hide. curl throws the
server's text away and prints curl: (67) Login denied, which is why people
spend an afternoon on a password that was never wrong.

Runs with npx zerosmtp-check, nothing to install. MIT licensed.

Checked against contributing.md: format [name](link) - Description., link
points at the tool's repository, description starts with a capital, ends with a
full stop, and does not begin with "A" or "An". Not already in the list.

Diff is one line.