use the zone file's basename as origin in dnssec tools

use the zone file's basename as origin in dnssec tools

In dnssec-signzone and dnssec-verify, if the zone origin is not
specified using the `-o` parameter, the default behavior is to try
to use the zone's file name as the origin. So, for example,
`dnssec-signzone -S example.com` or 'dnssec-verify 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` or
'dnssec-verify zones/example.com' will set the origin value
to `example.com`.