libcasper: reference system.fileargs service

Reviewed by: oshogbo
Fixes: cdd6ea94b0a4 ("libcasper: introduce cap_fileargs...")
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D41761

(cherry picked from commit 56e3123fc864b6d08ee5b3752538d2f9be52027b)
(cherry picked from commit 99f8ee196deb0ca2b2a19602a317ac61e9ca25a0)

Approved by: re (gjb)