tree-wide: Various forward header cleanups

- Make sure forward headers have the iwyu pragma to always keep them
- Make sure we always include the daemon specific forward header
  instead of shared-forward.h
- Remove shared-forward.h include where the daemon specific forward
  header is already included

src/sysupdate/sysupdate-forward.h

@@ -1,6 +1,8 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 #pragma once
 
+/* IWYU pragma: always_keep */
+
 #include "shared-forward.h" /* IWYU pragma: export */
 
 typedef struct Context Context;