Use shared configfile instead of custom policy.json path handling.
This updates ocipull to rely on signature.DefaultPolicy(), removes
explicit SignaturePolicyPath, and replaces trust's custom default-policy
path logic with common configfile code.
Replace hidden `--policypath` with --signature-policy` and require
it for `trust set` command instead of path resolution based on
configfile.
For `trust get`, the `--signature-policy` is optional.
Signed-off-by: Jan Kaluza <jkaluza@redhat.com>
54d61c2f02