fix: dev: Fix a bug with template filename reuse

When a zone filename is defined in `named.conf` which will be
written to by the server - i.e., for secondary or dynamically updated
zones - there is a test at configuration time to ensure that the
filename is non-unique.

This test is run before the zone is actually created, so a zone
configured using a template may not have had its filename expanded
yet.  This can cause a configuration to fail because, for example,
multiple zones appear to using the filename `$name.db`.

This has been fixed by adding a new function `dns_zone_expandzonefile()`
and calling it during the uniqueness check.

Merge branch 'each-template-file' into 'main'

See merge request isc-projects/bind9!11769