cp: Clarify an obscure comment.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: allanjude
Differential Revision: https://reviews.freebsd.org/D44805

(cherry picked from commit 64d6925d1901637125f9f739282e72c992657dc8)

cp: Additional sanity check.

Once we've successfully opened the file we've been asked to copy, check
that it's of the same type as FTS told us it was.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: allanjude, markj
Differential Revision: https://reviews.freebsd.org/D44806

(cherry picked from commit 9075d4cfad5b339aabdf8033623a2164898c2786)

cp: Use warnc().

MFC after: 1 week
Sponsored by: Klara, Inc.

(cherry picked from commit f070188c3ad6b87ee9ce220b21718333d1bd9d52)