Add a mechanism to record namespaces for synth-from-dnssec [v9_18]

When namespace is grafted on, the DNSSEC proofs for non existance need to come from that namespace and not a higher namespace. We add 3 function dns_view_sfd_add, dns_view_sfd_del and dns_view_sfd_find to add, remove and find the namespace that should be used when checking NSEC records.

dns_view_sfd_add adds a name to a tree, creating the tree if needed. If the name already existed in the tree the reference count is increased otherwise it is initalised to 1.

dns_view_sfd_del removes a reference to a name in the tree, if the count goes to 0 the node is removed.

dns_view_sfd_find returns the namespace to be used to entered name. If there isn't an enclosing name in the tree, or the tree does not yet exist, the root name is returned.

Access to the tree is controlled by a read/write lock.

(cherry picked from commit 3619cad1)

Closes #3402 (closed)