new: dev: Add constant-time DNS root-name checks

Checking whether a DNS name is the root could require walking the name
to count its labels or comparing it with the global root name. A new
dns_name_isroot() helper checks the root wire encoding directly,
distinguishes the empty name from the root, and replaces the existing
ad hoc checks.

Merge branch 'each-name-isroot' into 'main'

See merge request isc-projects/bind9!11920