Fix sending replies to PTR questions

Fix sending replies to PTR questions

When we receive PTR question it includes hostname (instance), e.g.:
mdnsd: parse_question (391): Q -> PTR lede._http._tcp.local

First of all we should check if it matches hostname we use before trying
to reply. Secondly service_reply expects service with domain appended
(without hostname/instance) so we need to strip received string out of
hostname before passing it.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: John Crispin <john@phrozen.org>