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/basic/basic-forward.h

@@ -0,0 +1,134 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+#pragma once
+
+/* IWYU pragma: always_keep */
+
+#include <errno.h>              /* IWYU pragma: export */
+#include <inttypes.h>           /* IWYU pragma: export */
+#include <limits.h>             /* IWYU pragma: export */
+#include <paths.h>              /* IWYU pragma: export */
+#include <stdarg.h>             /* IWYU pragma: export */
+#include <stdbool.h>            /* IWYU pragma: export */
+#include <stddef.h>             /* IWYU pragma: export */
+#include <stdint.h>             /* IWYU pragma: export */
+#include <sys/types.h>          /* IWYU pragma: export */
+#include <uchar.h>              /* IWYU pragma: export */
+
+#include "assert-util.h"        /* IWYU pragma: export */
+#include "cleanup-util.h"       /* IWYU pragma: export */
+#include "macro.h"              /* IWYU pragma: export */
+
+/* Generic types */
+
+typedef uint64_t usec_t;
+typedef uint64_t nsec_t;
+
+/* Libc/Linux forward declarations */
+
+struct dirent;
+struct ether_addr;
+struct fiemap;
+struct file_handle;
+struct glob_t;
+struct group;
+struct icmp6_hdr;
+struct in_addr;
+struct in6_addr;
+struct inotify_event;
+struct iovec;
+struct mount_attr;
+struct msghdr;
+struct passwd;
+struct pollfd;
+struct rlimit;
+struct sgrp;
+struct shadow;
+struct signalfd_siginfo;
+struct siphash;
+struct sockaddr;
+struct spwd;
+struct stat;
+struct statfs;
+struct statx_timestamp;
+struct statx;
+struct termios;
+struct tm;
+struct ucred;
+
+/* To forward declare FILE and DIR, we have to declare the internal struct names for them. Since these are
+ * used for C++ symbol name mangling, they're effectively part of the ABI and won't actually change. */
+typedef struct _IO_FILE FILE;
+typedef struct __dirstream DIR;
+
+/* 3rd-party library forward declarations */
+
+enum bpf_map_type;
+
+struct fdisk_context;
+struct fdisk_table;
+struct crypt_device;
+
+/* basic/ forward declarations */
+
+typedef void (*hash_func_t)(const void *p, struct siphash *state);
+typedef int (*compare_func_t)(const void *a, const void *b);
+typedef compare_func_t comparison_fn_t;
+typedef int (*comparison_userdata_fn_t)(const void *, const void *, void *);
+
+struct hash_ops;
+struct hw_addr_data;
+struct in_addr_data;
+struct iovec_wrapper;
+union in_addr_union;
+union sockaddr_union;
+
+typedef enum CGroupFlags CGroupFlags;
+typedef enum CGroupMask CGroupMask;
+typedef enum ChaseFlags ChaseFlags;
+typedef enum ExtractFlags ExtractFlags;
+typedef enum Glyph Glyph;
+typedef enum ImageClass ImageClass;
+typedef enum JobMode JobMode;
+typedef enum RuntimeScope RuntimeScope;
+typedef enum TimestampStyle TimestampStyle;
+typedef enum UnitActiveState UnitActiveState;
+typedef enum UnitDependency UnitDependency;
+typedef enum UnitType UnitType;
+
+typedef struct Hashmap Hashmap;
+typedef struct HashmapBase HashmapBase;
+typedef struct IteratedCache IteratedCache;
+typedef struct Iterator Iterator;
+typedef struct OrderedHashmap OrderedHashmap;
+typedef struct OrderedSet OrderedSet;
+typedef struct Set Set;
+
+typedef struct dual_timestamp dual_timestamp;
+typedef struct triple_timestamp triple_timestamp;
+typedef struct ConfFile ConfFile;
+typedef struct LockFile LockFile;
+typedef struct PidRef PidRef;
+typedef struct Prioq Prioq;
+typedef struct RateLimit RateLimit;
+typedef struct SocketAddress SocketAddress;
+
+/* Constants */
+
+/* We duplicate various commonly used constants here so we can keep most static inline functions without
+ * having to include the full header that provides these constants. */
+
+#define AT_FDCWD                -100
+#define AT_EMPTY_PATH           0x1000
+#define AT_SYMLINK_FOLLOW       0x400
+#define AT_SYMLINK_NOFOLLOW     0x100
+
+#define MODE_INVALID            ((mode_t) -1)
+
+#define UID_INVALID             ((uid_t) -1)
+#define GID_INVALID             ((gid_t) -1)
+
+#define USEC_INFINITY           ((usec_t) UINT64_MAX)
+#define NSEC_INFINITY           ((nsec_t) UINT64_MAX)
+
+/* MAX_ERRNO is defined as 4095 in linux/err.h. We use the same value here. */
+#define ERRNO_MAX               4095