DNS names are case-insensitive, but clients match extra records against
the lowercased query name, so records with mixed-case names (for example
"Printer.fritz.box" in an extra_records_path file) never resolved and
queries fell through to the global nameserver.
Normalize record names to lowercase where the records enter the tailcfg
DNS config, covering both dns.extra_records and extra_records_path.
Fixes#2782
