gpart: add libxo support for "show" subcommand + man page updates

Added libxo support to gpart show, also updated the man
pages for geom and gpart to show where you can expect
libxo formatted output.

PR: 290629
Sponsored by: ConnectWise
Reviewed by: asomers, mckusick, phil
Approved by: asomers (mentor)
Differential Revision: https://reviews.freebsd.org/D53950

(cherry picked from commit 4f809ffec69cd6ede3e7be9a5bc876b2e5931028)

geom_part: Fix format string issues

This fixes a segfault on i386 and armv7 and numerous style violations.

PR: 292008
Fixes: 4f809ffec69c ("gpart: add libxo support for "show" subcommand + man page updates")
Reviewed by: js
Differential Revision: https://reviews.freebsd.org/D54393

(cherry picked from commit faff7013187b03f4240af6f52f608e7ebafe19f3)