w: Complete libxo transition.

MFC after: 1 week
Sponsred by: Klara, Inc.
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D38172

(cherry picked from commit d90ff31ae5cd9ba073fbd6a7b2367b5209ea534e)

w: Unlike err(3), xo_err(3) won't accept a null format string.

MFC after: 1 week
X-MFC-with: d90ff31ae5cd
Sponsored by: Klara, Inc.
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D38192

(cherry picked from commit 6fc1bbbf62bda2f1b5be3a4be3d36bc964490ebe)