log: use logtail to log and upload sockstat logs

Switch to using logtail for logging sockstat logs. Always log locally
(on supported platforms), but disable automatic uploading.  Change
existing c2n sockstats request to trigger upload to log server and
return log ID.

Signed-off-by: Will Norris <will@tailscale.com>

ipn/ipnlocal/c2n.go

@@ -84,8 +84,13 @@ func (b *LocalBackend) handleC2N(w http.ResponseWriter, r *http.Request) {
 		}
 		writeJSON(res)
 	case "/sockstats":
+		if r.Method != "POST" {
+			http.Error(w, "bad method", http.StatusMethodNotAllowed)
+			return
+		}
 		w.Header().Set("Content-Type", "text/plain")
-		b.sockstatLogger.WriteLogs(w)
+		b.sockstatLogger.Flush()
+		fmt.Fprintln(w, b.sockstatLogger.LogID())
 	default:
 		http.Error(w, "unknown c2n path", http.StatusBadRequest)
 	}