refactor: merge File/FileReceive, add optional metadata transfer with -M flag

- Remove FileReceive struct; use File everywhere with nullable FileMetadata*
- Remove struct stat from File; file size lives in Data->size (data_create_reserve)
- Add FileMetadata struct (mode, uid, gid, mtime) sent conditionally over wire
- Add -M / --preserve flag to client
- Restore permissions, ownership, timestamps on disk write
- Wire format uses per-file present flag for metadata (zero overhead when off)

src/shared/utils.h

@@ -1,9 +1,12 @@
 #ifndef UTILS_H
 #define UTILS_H
 
+#include "file.h"
+
 void mkdir_r(char *path);
 char *str_dup(char *string);
 void to_disk(char *path, void *data, unsigned long long data_size);
+void file_restore_metadata(const char *path, FileMetadata *metadata);
 char *path_cat(char *path1, char *path2);
 
 #endif