IPv6 RA should respect and utilize kernel stable-privacy and tempaddr flags

[perkelix]

In its current form (10.0.6), dhcpcd ignores kernel settings and creates pseudo-private IPv6 addresses. If net.ipv6.conf.*.addr_gen_mode=3 or net.ipv6.conf.*.use_tempaddr=2 (Linux) are set by procps during bootup, dhcpcd ignores them.

This is undesirable for a number of reasons.

What would be a better implementation is to first try enabling stable-privacy and, if that fails, enable temporary addresses. The later has been implemented ages ago and will thus probably always succeed, and it would work well as a failsafe for the former, whose implementation is more recent on most kernels.

This implementation should also respect both options if they have already been configured during bootup.