netstat: Add missing "shutdown-ack-timer" key to libxo output

The key "shutdown-timer" was present twice in the libxo output. The second
occurence of "shutdown-time" has been renamed to "shutdown-ack-timer".

PR: 254672
Reviewed by: imp, zlei
Pull Request: https://github.com/freebsd/freebsd-src/pull/1331