startup error -> "[emerg] duplicate listen options" if 2nd IPv6 listener is enabled

[pgn]

Done.

Reading the docs, it's not clear to me -- or others apparently:

	http://kromey.us/2011/06/named-based-virtual-hosts-with-nginx-on-ipv6-423.html
	http://serverfault.com/questions/277653/nginx-name-based-virtual-hosts-on-ipv6

-- that ipv6only=on can only be specified once ...

While some 'additional parameters' on the listen line are restricted in
this manner, others -- e.g., ssl, spdy -- are not, and can appear
multiple times.

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,228250,228257#msg-228257