dnsmasq: fix dhcp-host entries with empty macs

dnsmasq: fix dhcp-host entries with empty macs

Due to improper localization of helper variables, "config host" entries
without a given mac address may inherit the mac address of a preceeding,
leading to invalid generated netive configuration.

Fix the issue by marking the "macs" and "tags" helper variables in
dhcp_host_add() local, avoiding the need for explicitely resetting them
with each invocation.

Reported-by: Russell Senior <russell@personaltelco.net>
Tested-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>
Signed-off-by: Jo-Philipp Wich <jo@mein.io>