Reimplement missing DNS providers that appear to have been removed in error

Closes #968964

An assortment of DNS providers have been removed from the Debian build of Lego; but the relevant documentation was not updated, the change was not noted in its commit message, and the above bug report has gone undisputed for 21 months so far. This likely indicates that the change was accidental, and should be reversed in order to restore the missing functionality and avoid confusing users.