device: introduce ipv6.temp-valid-lifetime and ipv6.temp-preferred-lifetime properties

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.