Various mostly bonjour related fixes

[Linus Lüssing]

Dear list,

With the following patches I was able to remove various issues I was
having with bonjour messaging and file transfer.

These patches were tested with two Debian testing VMs with one
network interface with avahi 0.6.30. Various combinations of IPv4,
IPv6 global and IPv6 link-local addresses assigned to their interface
were tested successfully.

They should probably be tested on some other operating systems as well
but I unfortunately do not have any other ones available at the moment.

Three more potential issues probably still exist (though unverified):

* IPv6 is currently being ignored on Windows XP (due to missing
  dualstack support - otherwise some more effort to maintain two separate
  sockets for IPv4+IPv6 would be needed) - see PATCH 2/4
* There is a low chance of a failing file transfer due to possible
  IPv6 link local address collisions - see PATCH 4/4.
* In case of using an mDNS reflector a file transfer might fail due to
  the attempt of connecting to an IPv6 link local address which is
  not reachable due to being more than one hop away
  (probably something similar to [427f3622bbeb4d441ed1d9578800c1d8dce7849f],
  ticket #13773, should be implemented for file transfers as well and
  maybe also sorting the offered IPv6 addresses by their reachability,
  prefering IPv6 global addresses the most, IPv6 link local addresses
  the least).


Cheers, Linus