zabbix: tweak server initscript and bring agentd to parity

zabbix: tweak server initscript and bring agentd to parity

* Genericize zabbix_server initscript.
* Add shellcheck overrides where needed.
* Add a 'never_root' config option (default true) that determines
  whether server starts as an unprivileged user or starts as root and
  drops to an unprivileged user (unless config is changed so it always
  runs as root).
* Bring zabbix_agentd initscript to parity with zabbix_server for the
  initscript and configuration.
* Tweak Makefile to reduce repetition

Signed-off-by: Daniel F. Dickinson <redacted>