rename `dns_view_findzonecut()` into `dns_view_bestzonecut()`

rename `dns_view_findzonecut()` into `dns_view_bestzonecut()`

`dns_view_findzonecut()` is used only in the context where the closest
name servers for a name need to be queried.  In the future, this API
will also return the glues (if known) for those name servers, as well
as (exclusively, if both NS and DELEG exist) the DELEG record.

To avoid ambiguities with other code flows using `dns_db_findzonecut()`,
`dns_view_findzonecut()` has been renamed into `dns_view_bestzonecut()`.