copy: extend copy_bytes() a bit

Optionally, when we copy between fds with simple read/write, let's
return any remaining data we already read into the buffer if write
fails. This is useful to allow callers to use the read data otherwise,
perhaps implementing a different fallback for copying.

src/basic/copy.h

@@ -25,6 +25,9 @@ int copy_tree(const char *from, const char *to, uid_t override_uid, gid_t overri
 int copy_tree_at(int fdf, const char *from, int fdt, const char *to, uid_t override_uid, gid_t override_gid, CopyFlags copy_flags);
 int copy_directory_fd(int dirfd, const char *to, CopyFlags copy_flags);
 int copy_directory(const char *from, const char *to, CopyFlags copy_flags);
-int copy_bytes(int fdf, int fdt, uint64_t max_bytes, CopyFlags copy_flags);
+int copy_bytes_full(int fdf, int fdt, uint64_t max_bytes, CopyFlags copy_flags, void **ret_remains, size_t *ret_remains_size);
+static inline int copy_bytes(int fdf, int fdt, uint64_t max_bytes, CopyFlags copy_flags) {
+        return copy_bytes_full(fdf, fdt, max_bytes, copy_flags, NULL, NULL);
+}
 int copy_times(int fdf, int fdt);
 int copy_xattr(int fdf, int fdt);