Fix default IANA root zone mirror configuration

Fix default IANA root zone mirror configuration

Commit 4555a319345ba96f8ba4b56b5423a865f3f40450 renamed the top-level
"primaries" block in bin/named/config.c to "remote-servers".  This
configuration block lists the primary servers used for an IANA root zone
mirror when no primary servers are explicitly specified for it in the
configuration.  However, the relevant part of the named_zone_configure()
function only looks for a top-level "primaries" block and not for any of
its synonyms.  As a result, configuring an IANA root zone mirror with
just:

    zone "." {
        type mirror;
    };

now results in a cryptic fatal error on startup:

    loading configuration: not found
    exiting (due to fatal error)

Fix by using the correct top-level block name in named_zone_configure().

(cherry picked from commit 010d2eb4360afd5f4049491f5e73e25003895106)