net-mgmt/zabbix5-server: Update to 5.0.15

Summary:

• Update to 5.0.15
• Reformat a bit according to portclippy and portfmt
• Rework pidfile and logfile handling (now under /var/{run,log}/zabbix)
• Provide sample config for agentd
• Changelog: https://www.zabbix.com/rn/rn5.0.15

PR: 258076
Reviewed by: pg@pakhom.spb.ru (maintainer)
Differential Revision: https://reviews.freebsd.org/D31845