resolved: fix "in-between" logic when boundaries are equal (#7590)

resolved: fix "in-between" logic when boundaries are equal (#7590)

This changes dns_name_between() to deal properly with checking whether B
is between A and C if A and C are equal. Previously we simply returned
-EINVAL in this case, refusing checking. With this change we correct
behaviour: if A and C are equal, then B is "between" both if it is
different from them. That's logical, since we do < and > comparisons, not
<= and >=, and that means that anything "right of A" and "left of C"
lies in between with wrap-around at the ends. And if A and C are equal
that means everything lies between, except for A itself.

This fixes handling of domains using NSEC3 "white lies", for example the
.it TLD.

Fixes: #7421