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
MFC after: 1 week
Sponsored by: ConnectWise
Reviewed by: asomers, mckusick, phil
Approved by: asomers (mentor)
Differential Revision: https://reviews.freebsd.org/D53950