Enable IPv6 Privacy Extensions by default

[Tijl Coosemans]

Hi,

I recently got a new modem/router from my ISP that supports IPv6.  Added
ifconfig_em0_ipv6="inet6 accept_rtadv" and rtsold_enable="YES" to
/etc/rc.conf like the handbook says and now all my FreeBSD systems have
an IPv6 address. \o/

I also added these lines to /etc/sysctl.conf to enable temporary
addresses:

net.inet6.ip6.use_tempaddr=1
net.inet6.ip6.prefer_tempaddr=1

Shouldn't these be enabled by default?  There was a proposal 9 years ago
that didn't get any objections but it seems it wasn't committed:
https://lists.freebsd.org/pipermail/freebsd-net/2008-June/018381.html

If there are no objections, I'll make the change in a week or so.