geom: Document existing --libxo support in the manual pagesHEADmain

The standard list and status subcommands of every GEOM class emit
through libxo, and so does geom -p, but only geom.8 (list, status) and
gpart.8 (show) mentioned it.  Mark the list and status forms with
--libxo in all class manual pages, add the option description, and
cross-reference xo_options(7).  Also mark geom -p in geom.8.

Only forms whose output already goes through libxo are marked;
class-specific verbs such as gmirror dump still print directly and are
left alone.

Reviewed by:	adrian, bcr, carlavilla, des
MFC after:	3 days
Differential Revision:	https://reviews.freebsd.org/D59574

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