Multiple copies of the default signature policy path

[dfr]

Recently, while working on porting podman system tests to FreeBSD, I noticed that the default location of the signature policy file is specified in multiple places:

• c/image/signature has builtinDefaultPolicyPath which is used by DefaultPolicy (but not exported)
• c/podman/pkg/trust has a function DefaultPolicyPath with a copy of the policy path in its implementation
• c/common/pkg/config exports DefaultSignaturePolicyPath as well as including its copy in the value returned by the function DefaultConfig

It would be nice to reduce that number down to one but it seems that the right way to do that is to somehow merge podman's pkg/trust with image's signature which is hard (see #1725).