hints: merge RRs instead of replacing them

hints: merge RRs instead of replacing them

We had this behavior in 5.x.
Lua level: affects hints.set() and hints['name'] and hints.add_hosts()
YAML level: /local-data/addresses and /local-data/addresses-files

I considered various approaches when writing this.  This one won because
in /etc/hosts like files a name can be repeated with arbitrary lines
in between, and users can reasonably expect it to collect all addresses.