Fix sysconfdir path in man pages

Some man pages have hardcoded values for the default configuration file paths. This path is configurable through the "sysconfdir" variable, and the manpages should perform the proper substitutions whenever the default value is changed.

This patch uses the already existing templating system for building the docs to perform such substitutions.

This patch has been submitted upstream at https://gitlab.isc.org/isc-projects/bind9/-/issues/2717 (it is an issue instead of an MR as per the project README file's instructions)