Summary:
Add the defines for the NNTP/NNTPS strings and ports;, implement the
basic conversation to get the capabilities of the server (if supported),
to know whether TLS is supported, and which authentication methods are
available.

Test Plan:
Run the `servertest` with some NNTP server, e.g. news.gmane.org,
nntp.perl.org, news.mozilla.org.

Reviewers: #kde_pim, mlaurent

Reviewed By: #kde_pim, mlaurent

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D6977