Given three DNS names this function indicates if the second argument lies
strictly between the first and the third according to the canonical DNS
name order. Note that the order is circular, so the last name is
considered to be before the first.
The canonical DNS name ordering considers the rightmost label the most significant,
we were considering it the least significant. This is important when implementing
NSEC, which relies on the correct order.
Intended to be called repeatedly, and returns then successive unescaped labels
from the most to the least significant (left to right).
This is slightly inefficient as it scans the string three times (two would be
sufficient): once to find the end of the string, once to find the beginning
of each label and lastly once to do the actual unescaping. The latter two
could be done in one go, but that seemed unnecessarily convoluted.
