basic,shared: move a bunch of files to src/shared/

The goal is to move everything that requires selinux or smack
away from src/basic/. This means that src/basic/label.[ch] must move,
which implies btrfs-util.[ch], copy.[ch], and a bunch of other files
which form a cluster of internal use.

This is just moving text around, so there should be no functional difference.

test-blockdev-util is new, because path_is_encrypted() is moved to
blockdev-util.c, and so far we didn't have any tests for code there.

src/shared/rm-rf.h

@@ -0,0 +1,51 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+#pragma once
+
+#include <sys/stat.h>
+
+#include "alloc-util.h"
+#include "errno-util.h"
+
+typedef enum RemoveFlags {
+        REMOVE_ONLY_DIRECTORIES = 1 << 0, /* Only remove empty directories, no files */
+        REMOVE_ROOT             = 1 << 1, /* Remove the specified directory itself too, not just the contents of it */
+        REMOVE_PHYSICAL         = 1 << 2, /* If not set, only removes files on tmpfs, never physical file systems */
+        REMOVE_SUBVOLUME        = 1 << 3, /* Drop btrfs subvolumes in the tree too */
+        REMOVE_MISSING_OK       = 1 << 4, /* If the top-level directory is missing, ignore the ENOENT for it */
+        REMOVE_CHMOD            = 1 << 5, /* chmod() for write access if we cannot delete or access something */
+        REMOVE_CHMOD_RESTORE    = 1 << 6, /* Restore the old mode before returning */
+} RemoveFlags;
+
+int unlinkat_harder(int dfd, const char *filename, int unlink_flags, RemoveFlags remove_flags);
+int fstatat_harder(int dfd,
+                const char *filename,
+                struct stat *ret,
+                int fstatat_flags,
+                RemoveFlags remove_flags);
+
+int rm_rf_children(int fd, RemoveFlags flags, struct stat *root_dev);
+int rm_rf(const char *path, RemoveFlags flags);
+
+/* Useful for usage with _cleanup_(), destroys a directory and frees the pointer */
+static inline char *rm_rf_physical_and_free(char *p) {
+        PROTECT_ERRNO;
+
+        if (!p)
+                return NULL;
+
+        (void) rm_rf(p, REMOVE_ROOT|REMOVE_PHYSICAL|REMOVE_MISSING_OK|REMOVE_CHMOD);
+        return mfree(p);
+}
+DEFINE_TRIVIAL_CLEANUP_FUNC(char*, rm_rf_physical_and_free);
+
+/* Similar as above, but also has magic btrfs subvolume powers */
+static inline char *rm_rf_subvolume_and_free(char *p) {
+        PROTECT_ERRNO;
+
+        if (!p)
+                return NULL;
+
+        (void) rm_rf(p, REMOVE_ROOT|REMOVE_PHYSICAL|REMOVE_SUBVOLUME|REMOVE_MISSING_OK|REMOVE_CHMOD);
+        return mfree(p);
+}
+DEFINE_TRIVIAL_CLEANUP_FUNC(char*, rm_rf_subvolume_and_free);