sockstat: complete libxo integration and improve formatting

Fixed missing field names in xo_emit() calls for xolint compliance, treated
XO_STYLE_HTML like XO_STYLE_TEXT to apply space-padded formatting for
human-readable output, and updated usage() to indicate that --libxo accepts
arguments.

Reported by: Phil Shafer <phil@juniper.net>
Sponsored by: Google, LLC (GSoC 2025)
MFC after: 2 weeks
Pull Request: https://github.com/freebsd/freebsd-src/pull/1806
Reviewed by: asomers