avahi: enable disallow-other-stacks option by default.

This is required to make this work in kernels < 3.9 (kernel-rpi is 3.6.11 currently),
which contains support for SO_REUSEPORT.

See http://stackoverflow.com/questions/14388706/socket-options-so-reuseaddr-and-so-reuseport-how-do-they-differ-do-they-mean-t
for more info.