net-mgmt/mk-livestatus: enable Nagios4 support by default

• ../UPDATING
  
  add note regarding default options being changed.
• Makefile
  
  adopt the port; update '1.2.8p22' to '1.2.8p24'; enable Nagios 4.x as the default supported version; RUN_DEPENDS uses respective Nagios package as dependency; delete unnecessary ${RM} -r api/perl; new option now provides mk-livestatus' Perl API; clean USES variable; built just fine.
• pkg-message
  
  update documentation's URL (linked the one with text in English); add clear note that Nagios' event broker support needs to be enabled.

Approved by: rene (mentor)
Differential Revision: https://reviews.freebsd.org/D18513