The fallthrough happened to work in controlclient already due to the
/etc/os-release PRETTY_NAME default, but make it explicit so it
doesn't look like an accident.
Also add it to version/distro, even though nothing needs it yet.
For now. Get it working again so it's not stuck on 0.98.
Subnet relay can come later.
Updates #451
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
