Respect DO_NOT_TRACK environment variable

[zomglings]

See: https://consoledonottrack.com/

We have already implemented BUGGER_OFF=1. We should just add DO_NOT_TRACK=1 as well with the same functionality.

We have to do this in every client library.

[miraculixx]

The default should be to assume BUGGER_OFF=true.

One easy way to avoid large GDPR fines is to always get permission from your users before using their personal data. Source: https://gdpr.eu/gdpr-consent-requirements/