libcasper: fix cap_rights_init usage

Capability rights passed to cap_rights_* are not simple bitmaks and
cannot be ORed together in general (although it will work for certain
subsets of rights).

PR: 277057
Fixes: faaf43b2a750 ("fileargs: add tests")
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 3733d82c4deb49035a39e18744085d1e3e9b8dc5)