w: don't truncate if we're writing libxo json/xml

If we're writing structured output (i.e. json or xml) we shouldn't worry
about terminal width, and instead always output full width information.

This means that, for example, if we're called from crontab with 'w
--libxo json' we'll provide full the command field rather than
pointlessly truncating it.

Suggested by: Phil Shafer
Event: Aberdeen Hackathon 2022
Differential Revision: https://reviews.freebsd.org/D25013