pw: Use copy_file_range() when copying skeleton files

As a bonus, check for I/O errors and print them to standard error.

Reviewed by: bapt, emaste
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D52588

(cherry picked from commit a03d150a3d44c49344ebd0862defd1a5ad1f524e)