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>

health/healthmsg/healthmsg.go

@@ -0,0 +1,14 @@
+// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package healthmsg contains some constants for health messages.
+//
+// It's a leaf so both the server and CLI can depend on it without bringing too
+// much in to the CLI binary.
+package healthmsg
+
+const (
+	WarnAcceptRoutesOff = "Some peers are advertising routes but --accept-routes is false"
+	TailscaleSSHOnBut   = "Tailscale SSH enabled, but " // + ... something from caller
+)