we should enable RFC7217 by default

Hi everyone,

With `net.inet6.ip6.use_stableaddr` now available, I believe we should 
enable it by default in CURRENT at least.
As you may already know, we currently use the EUI64 method for 
generating stable IPv6 addresses, which has serious privacy issues.

IMHO, trying to maintain backward compatibility defeats the purpose of a 
privacy RFC.

To be clear, we don't want to change the ip addresses of existing 
servers. However, it's reasonable for users to expect changes during a 
major upgrade (15 -> 16), a fresh install of a new major release, or 
living on CURRENT.
So, for obvious reasons, changing the default value would not be MFCed.

What do you think?

-- 
Pouria