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>

types/netmap/netmap.go

@@ -84,6 +84,16 @@ type NetworkMap struct {
 	UserProfiles map[tailcfg.UserID]tailcfg.UserProfile
 }
 
+// AnyPeersAdvertiseRoutes reports whether any peer is advertising non-exit node routes.
+func (nm *NetworkMap) AnyPeersAdvertiseRoutes() bool {
+	for _, p := range nm.Peers {
+		if len(p.PrimaryRoutes) > 0 {
+			return true
+		}
+	}
+	return false
+}
+
 // PeerByTailscaleIP returns a peer's Node based on its Tailscale IP.
 //
 // If nm is nil or no peer is found, ok is false.