[PATCH] dns_multicast_local configuration option

Wider testing in production (by Ralf Hildenbrandt) has identified that
the mDNS support added in 3.4 can produce a large number amount of
multicast traffic from .arpa lookups even if mDNS is not setup on the
local network.

Add a configuration option dns_multicast_local to turn mDNS lookups
ON/OFF and remove the DNS lag introduced by these useless lookups.

FUTURE WORK:
  It would be nice to implement a fast mechanism for determining whether
the PTR lookups are for LAN or WAN addresses
  to decide better whether to send the query as mDNS.

Amos