cat: add CAP_SEEK capability

The copy_file_range might require that.

Reviewed by: emaste, theraven, kib, markj
Differential Revision: https://reviews.freebsd.org/D41967

(cherry picked from commit 64c6327738fc24bb34b9f40ff1c223ab460ef637)
(cherry picked from commit 5f527dbe41b804d3fc6a789c5def25b25a44a3a0)