tree-wide: Introduce sd-forward.h and shared-forward.h headers

Let's not leak details from src/shared and src/libsystemd into
src/basic, even though you can't actually do anything useful with
just forward declarations from src/shared.

The sd-forward.h header is put in src/libsystemd/sd-common as we
don't have a directory for shared internal headers for libsystemd
yet.

Let's also rename forward.h to basic-forward.h to keep things
self-explanatory.

src/nspawn/nspawn-bind-user.h

@@ -1,5 +1,5 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 
-#include "forward.h"
+#include "shared-forward.h"
 
 int bind_user_setup(const MachineBindUserContext *c, const char *root);

src/nspawn/nspawn-cgroup.h

@@ -1,7 +1,7 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 #pragma once
 
-#include "forward.h"
+#include "shared-forward.h"
 #include "nspawn-settings.h"
 
 int create_subcgroup(

src/nspawn/nspawn-expose-ports.h

@@ -1,7 +1,7 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 #pragma once
 
-#include "forward.h"
+#include "shared-forward.h"
 #include "list.h"
 
 typedef struct ExposePort {

src/nspawn/nspawn-mount.h

@@ -1,7 +1,7 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 #pragma once
 
-#include "forward.h"
+#include "shared-forward.h"
 #include "volatile-util.h"
 
 typedef enum MountSettingsMask {

src/nspawn/nspawn-network.h

@@ -3,7 +3,7 @@
 
 #include <linux/if.h>
 
-#include "forward.h"
+#include "shared-forward.h"
 
 int test_network_interfaces_initialized(char **iface_pairs);
 int resolve_network_interface_names(char **iface_pairs);

src/nspawn/nspawn-oci.h

@@ -1,7 +1,7 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 #pragma once
 
-#include "forward.h"
+#include "shared-forward.h"
 #include "nspawn-settings.h"
 
 int oci_load(FILE *f, const char *path, Settings **ret);

src/nspawn/nspawn-register.h

@@ -1,7 +1,7 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 #pragma once
 
-#include "forward.h"
+#include "shared-forward.h"
 #include "nspawn-settings.h"
 
 int register_machine(

src/nspawn/nspawn-seccomp.h

@@ -1,6 +1,6 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 #pragma once
 
-#include "forward.h"
+#include "shared-forward.h"
 
 int setup_seccomp(uint64_t cap_list_retain, char **syscall_allow_list, char **syscall_deny_list);

src/nspawn/nspawn-settings.h

@@ -6,7 +6,7 @@
 #include "capability-util.h"
 #include "conf-parser-forward.h"
 #include "cpu-set-util.h"
-#include "forward.h"
+#include "shared-forward.h"
 #include "rlimit-util.h"
 #include "seccomp-util.h"
 #include "volatile-util.h"

src/nspawn/nspawn-setuid.h

@@ -1,7 +1,7 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 #pragma once
 
-#include "forward.h"
+#include "shared-forward.h"
 
 int change_uid_gid_raw(uid_t uid, gid_t gid, const gid_t *supplementary_gids, size_t n_supplementary_gids, bool chown_stdio);
 int change_uid_gid(const char *user, bool chown_stdio, char **ret_home);

src/nspawn/nspawn-stub-pid1.h

@@ -3,6 +3,6 @@
 
 #include "sd-id128.h"
 
-#include "forward.h"
+#include "shared-forward.h"
 
 int stub_pid1(sd_id128_t uuid);

src/nspawn/nspawn.h

@@ -1,7 +1,7 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 #pragma once
 
-#include "forward.h"
+#include "shared-forward.h"
 
 int userns_lchown(const char *p, uid_t uid, gid_t gid);
 int userns_mkdir(const char *root, const char *path, mode_t mode, uid_t uid, gid_t gid);