Convert to use libxo.

Obtained from:  Phil Shafer <phil@juniper.net>
Sponsored by:   Juniper Networks, Inc.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274213 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f
Document that df(1) supports libxo(3).


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274214 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f