nl(1): Capsicumise the utility

Signed-off-by: Faraz Vahedi <kfv@kfv.io>
Reviewed by: markj, oshogbo
MFC after: 1 week
Pull Request: https://github.com/freebsd/freebsd-src/pull/1465

(cherry picked from commit dfd52321b7beba716fa2bdd4f54e57e9ac806e96)