When IPv6 privacy extensions are enabled, by default temporary addresses
have a valid lifetime of 1 week and a preferred lifetime of 1 day.
That's far too long for privacy-conscious users, some of whom want a new
address once every few seconds. Add connection options that correspond
to /proc/sys/net/ipv6/conf/*/temp_valid_lft and
/proc/sys/net/ipv6/conf/*/temp_prefered_lft to allow configuring the
address rotation time on a per-connection basis.

The new properties are defined as 32-bit signed integers to match the
sysctl parameters which are also signed, although currently only
positive numbers are valid.