polkit: Show user name, picture

[ids1024]

Identity contains the UID polkit is authenticating as. If run as a non-admin user, it is prompting for the admin password, so not displaying this information may be confusing on multi-user systems.

I'm not sure what to do here if identity_kind isn't unix-user (I guess that would apply on a system configured for something like Kerberos for authentication?).

I also need to test how things like 2fa configured with PAM are handled. Presumably polkit-agent-helper-1 will prompt for those.