Change wording to indicate may not XFR instead of cannot

Change wording to indicate may not XFR instead of cannot

When a client has no permission to AXFR a zone from a server, the following diagnostics are logged:

```
AXFR of domain 'example' denied: client IP 10.0.0.1 has no permission
AXFR of domain 'example' failed: 10.0.0.1 cannot request AXFR
```

I feel the word _cannot_ is incorrect and should be _may not_ to indicate permission.