debian: Rename debian directory to debtpl (fixes #3099)

To keep it out of the way for actual, real, Debian packagers

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3195

debtpl/syncthing/postinst

@@ -0,0 +1,6 @@
+#!/bin/bash
+set -euo pipefail
+
+if [[ ${1:-} == configure ]]; then
+	pkill -HUP -x syncthing || true
+fi