Convert to use libxo.

Obtained from:	Phil Shafer <phil@juniper.net>
Sponsored by:	Juniper Networks, Inc.
Document that wc(1) supports libxo(3).
In alloc_pread() and kern_pread(), print errors only when DEBUG is
defined. An error is not fatal and is supposed to be handled by the
caller.

Obtained from:	Juniper Networks, Inc.