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/shared/shared-forward.h

@@ -0,0 +1,72 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+#pragma once
+
+/* IWYU pragma: always_keep */
+
+#include "sd-forward.h"         /* IWYU pragma: export */
+
+typedef int (*copy_progress_bytes_t)(uint64_t n_bytes, uint64_t bytes_per_second, void *userdata);
+typedef int (*copy_progress_path_t)(const char *path, const struct stat *st, void *userdata);
+
+struct local_address;
+struct in_addr_prefix;
+struct in_addr_full;
+
+typedef enum AskPasswordFlags AskPasswordFlags;
+typedef enum BootEntryTokenType BootEntryTokenType;
+typedef enum BusPrintPropertyFlags BusPrintPropertyFlags;
+typedef enum BusTransport BusTransport;
+typedef enum CatFlags CatFlags;
+typedef enum CertificateSourceType CertificateSourceType;
+typedef enum DnsCacheMode DnsCacheMode;
+typedef enum DnsOverTlsMode DnsOverTlsMode;
+typedef enum DnssecMode DnssecMode;
+typedef enum Fido2EnrollFlags Fido2EnrollFlags;
+typedef enum KeySourceType KeySourceType;
+typedef enum LabelFixFlags LabelFixFlags;
+typedef enum MountInNamespaceFlags MountInNamespaceFlags;
+typedef enum NamePolicy NamePolicy;
+typedef enum OutputFlags OutputFlags;
+typedef enum OutputMode OutputMode;
+typedef enum PagerFlags PagerFlags;
+typedef enum PatternCompileCase PatternCompileCase;
+typedef enum RemoveFlags RemoveFlags;
+typedef enum ResolveSupport ResolveSupport;
+typedef enum TPM2Flags TPM2Flags;
+typedef enum Tpm2Support Tpm2Support;
+typedef enum Tpm2UserspaceEventType Tpm2UserspaceEventType;
+typedef enum UnitFileFlags UnitFileFlags;
+typedef enum UnitFilePresetMode UnitFilePresetMode;
+typedef enum UnitFileState UnitFileState;
+typedef enum UserDBFlags UserDBFlags;
+typedef enum UserRecordLoadFlags UserRecordLoadFlags;
+typedef enum UserStorage UserStorage;
+
+typedef struct Bitmap Bitmap;
+typedef struct BPFProgram BPFProgram;
+typedef struct BusObjectImplementation BusObjectImplementation;
+typedef struct CalendarSpec CalendarSpec;
+typedef struct Condition Condition;
+typedef struct ConfigSection ConfigSection;
+typedef struct ConfigTableItem ConfigTableItem;
+typedef struct CPUSet CPUSet;
+typedef struct FDSet FDSet;
+typedef struct Fido2HmacSalt Fido2HmacSalt;
+typedef struct GroupRecord GroupRecord;
+typedef struct Image Image;
+typedef struct ImagePolicy ImagePolicy;
+typedef struct InstallInfo InstallInfo;
+typedef struct LookupPaths LookupPaths;
+typedef struct LoopDevice LoopDevice;
+typedef struct MachineBindUserContext MachineBindUserContext;
+typedef struct MachineCredentialContext MachineCredentialContext;
+typedef struct MountOptions MountOptions;
+typedef struct OpenFile OpenFile;
+typedef struct Pkcs11EncryptedKey Pkcs11EncryptedKey;
+typedef struct Table Table;
+typedef struct Tpm2Context Tpm2Context;
+typedef struct Tpm2Handle Tpm2Handle;
+typedef struct Tpm2PCRValue Tpm2PCRValue;
+typedef struct UnitInfo UnitInfo;
+typedef struct UserRecord UserRecord;
+typedef struct VeritySettings VeritySettings;