zabbix: deduplicate and reorganize package defines

zabbix: deduplicate and reorganize package defines

In preparation for further changes, deduplicate package definitions,
and reorganize them. At the same time make use of provides to ensure
both existing names are preserved, and that it is possible to be
specific about the variant of the package one wants.

Also, condense the package conffiles, install, postinst, etc handling.
This is more maintainable (less copy and paste and less to modify).

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