fix: usr: Use the zone file's basename as origin in DNSSEC tools

In `dnssec-signzone` and `dnssec-verify`, when the zone origin is not specified using the `-o` parameter, the default behavior is to try to sign using the zone's file name as the origin. So, for example, `dnssec-signzone -S example.com` will work, so long as the file name matches the zone name.

This now also works if the zone is in a different directory. For example, `dnssec-signzone -S zones/example.com` will set the origin value to `example.com`.

Closes #5678

Merge branch '5678-signzone-basename' into 'main'

See merge request isc-projects/bind9!11360