New port: net-mgmt/check_mk_agent: Nagios-compatible agent for the Check_MK monitoring

Check_MK is a software for effective IT monitoring. Easy installation,
best-in-class scalability and a broad range of features are just some of the
reasons why more and more users become Check_MK fans.

WWW: http://mathias-kettner.com/check_mk.html

Reviewed by: mat
Approved by: mat (mentor)
Sponsored by: BALLY WULFF Games & Entertainment GmbH
Differential Revision: https://reviews.freebsd.org/D18891