Nsec dname cmp zero

This "solves" the situation when records in zone are wrongly ordered because of knot_dname_lf with connection to \000.

The user is warned when creating NSEC tree, when owner with \000 is present.

NSEC tree creation/update fails, if this directly affects zone ordering.

This does not solve many situations when there is collision of names after knot_dname_lf applied!