Previously the client had heuristics to calculate which DNS search domains
to set, based on the peers' names. Unfortunately that prevented us from
doing some things we wanted to do server-side related to node sharing.
So, bump MapRequest.Version to 9 to signal that the client only uses the
explicitly configured DNS search domains and doesn't augment it with its own
list.
Updates tailscale/corp#1026
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
