fix: usr: Prevent ddns-confgen self-domain update-policy injection

In `ddns-confgen`, the user-supplied domain name to be updated was not properly escaped and put inside a double-quoted string when generating the example code fragment.  This has been corrected.

Closes #6072

Merge branch '6072-ddns-confgen-self-domain-update-policy-injection' into 'main'

See merge request isc-projects/bind9!12409