net-mgmt/nsca-ng: Update to 1.6

This is not broken anymore in 12 and 13

ChangeLog:
https://github.com/weiss/nsca-ng/blob/master/NEWS#changes-in-release-16-2019-03-19

Accumulated changes from 1.4:

• Work around TLSv1.3 PSK bug in (at least) OpenSSL 1.1.1b.
• Fix static linking against recent OpenSSL versions.
• Update our copy of libev from version 4.22 to version 4.25. The new release provides a few platform compatibilty fixes and small enhancements.
• Apply a few minor improvements.
• Include an experimental NSCA-ng client module for Perl.
• Link against libsystemd instead of libsystemd-daemon when using systemd version 210 or newer, where the latter library has been merged into the former.
• Include the systemd.service(5) and systemd.socket(5) files with the release tarball.
• Update our copy of libev from version 4.15 to version 4.22. The new release provides a number of bug fixes and small enhancements.

While here, update WWW in pkg-descr

PR: 247145
Submitted by: knan-bfo@modirum.com
Reviewed by: pi, fernape
Approved by: alexander@sulfrian.net (maintainer, timeout > 1 month)
Differential Revision: https://reviews.freebsd.org/D25208