IPv6 Unique Local Addresses are sometimes used with Network
Prefix Translation to reach the Internet. In that respect
their use is similar to the private IPv4 address ranges
10/8, 172.16/12, and 192.168/16.
Treat them as sufficient for AnyInterfaceUp(), but specifically
exclude Tailscale's own IPv6 ULA prefix to avoid mistakenly
trying to bootstrap Tailscale using Tailscale.
This helps in supporting Google Cloud Run, where the addresses
are 169.254.8.1/32 and fddf:3978:feb1:d745::c001/128 on eth1.
Signed-off-by: Denton Gentry <dgentry@tailscale.com>