Turn off ipv6only on systems where it is on by default
Systems vary in their treatment of the IPV6_V6ONLY option; the RFC says it should be on by default (apparently, only Linux deviates). For CherryPy, I propose that it is more useful if the default is off, when the user specifies :: as the bind_address - this supposedly means "all incoming connections". The attached patch makes sure the option is disabled on systems that have the option in the first place.
Reported by firstname.lastname@example.org