ntpclient v2018_176

The Stuttgart release. Curated by Joachim Nilsson.

Changes

• NTP server argument is now optional, default: pool.ntp.org
• Restore .tar.gz archives for make dist and release
• Make .deb file part of release
• Cleanup and simplify README
• Renamed doc/ to docs/
• Moved TODO and CONTRIBUTING.md to docs/

Fixes

• Issue #2: Missing fallback to IPv4 on systems with IPv6 disabled