DNMRFC: system token support, variant 1

[mcg1969]

This addition allows anaconda_anon_usage to be used to help disaggregate usage from an organization while still preserving per-user anonymity.

To use it, an administrator would deposit a short token in a system location, presumably with MDM software.

• Unix: /etc/conda/aau_token or /var/lib/conda/aau_token
• Windows: C:\ProgramData\conda\aau_token

When the client_token is retrieved, it also retrieves this system token and ensures that the client token is prefixed by the system token. The total token length is fixed at 22 characters; that hasn't changed. So for instance, if the organization token is 6 characters, the randomness is reduced to 16 characters. If for some reason the client token is tampered with or removed, or the system-installed prefix is changed, anaconda_anon_usage will re-generate it with the proper prefix.

If the system token is not present, as it will be in all normal scenarios, the full random token is generated instead. In fact, this code is slightly more random than the previous implementation. 22 byte64 encoded characters can hold 16.5 bytes worth of randomness, but we were only generating 16 before; now we are generating 17.

[mcg1969]

Closing in favor of #110