Add optional ipv6 support.

This is disabled by default. If enough slap me, I'll happily turn it
on. I suspect most servers are not ipv6 enabled still, and I'm
afraid enabling it by default could cause issues in those cases.

If enabled it installs lua-socket from the prosody apt repository.

See http://blog.prosody.im/prosody-0-9-0rc3-available-for-testing/
for more information