Rename packages to `zabbix-agent-pgsql`

[cavaliercoder]

The current name makes sense as standard name for a shared module, but not for the scope of the monitoring package (deb/rpm/other).

The package should be named zabbix-agent-pgsql more inline with Zabbix's naming convention and is more apparent as an extension to the Zabbix agent.

The package could then contain multiple templates, documentation, etc. for multiple monitoring scenarios related to PostgreSQL.

[cavaliercoder]

Or maybe zabbix-pgsql or zabbix-module-pgsql or zabbix-mod-pgsql... needs to be consistent with other modular packages and reusable by other authors.