cmd/tailscale: make "up", "status" warn if routes and --accept-routes off

Example output:

    # Health check:
    #     - Some peers are advertising routes but --accept-routes is false

Also, move "tailscale status" health checks to the bottom, where they
won't be lost in large netmaps.

Updates #2053
Updates #6266

Change-Id: I5ae76a0cd69a452ce70063875cd7d974bfeb8f1a
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscale/depaware.txt

@@ -51,6 +51,7 @@ tailscale.com/cmd/tailscale dependencies: (generated by github.com/tailscale/dep
         tailscale.com/derp/derphttp                                  from tailscale.com/net/netcheck
         tailscale.com/disco                                          from tailscale.com/derp
         tailscale.com/envknob                                        from tailscale.com/cmd/tailscale/cli+
+        tailscale.com/health/healthmsg                               from tailscale.com/cmd/tailscale/cli
         tailscale.com/hostinfo                                       from tailscale.com/net/interfaces+
         tailscale.com/ipn                                            from tailscale.com/cmd/tailscale/cli+
         tailscale.com/ipn/ipnstate                                   from tailscale.com/cmd/tailscale/cli+