restrict encoding/decoding credentials with a given key based on the request UID/GID

[dun]

Add the ability to restrict encoding and decoding of credentials with a given key based on the request UID and/or GID. This would help mitigate the PLATYPUS power side-channel attack by an unprivileged user. It would allow the administrator to reduce the attack surface of a given key to a trusted set of users and/or groups.

This must be specified in the configuration file (#58).