rpcd-mod-luci: refactor host hint gathering

 - Do not incorrectly put the IP addresses from the rrdns reply into the
   first hint, instead distribute hostnames to the appropriate hints.

 - Avoid returning stale lease information from previously parsed records
   by clearing the static struct before each parse attempt.

 - Improve existing checks for empty MAC addresses and unspecified IPv6
   addresses.

Fixes: #5126
Fixes: 87bcf9e923 ("rpcd-mod-luci: Return array of addresses in getHostHints")
Signed-off-by: Jo-Philipp Wich <jo@mein.io>