devices: --devices/--specials/-D/--copy-devices/--write-devices

Recreate char/block nodes via mknodat (privilege-gated, EPERM->warn+skip) and
FIFOs via mkfifoat; new STATUS_SPECIAL frame + validated rdev; sockets skipped;
-copy-devices copies st_size; -write-devices O_NOFOLLOW+O_NONBLOCK warn+skip.
preserve_specials/copy_devices/write_devices cross the wire. PROTOCOL_VERSION
2.12.0->2.13.0. Review fixes: -m source-removal keeps recreated specials, FIFO
ENXIO skip, rdev bounds at chunk_deserialize, STATUS_ERROR on receive branch,
scanner_prepare_special dedup.

src/shared/file.c

@@ -114,6 +114,9 @@ File* file_create(const char* path) {
   file->link_group = 0;
   file->link_first = false;
   file->hardlink_target = NULL;
+  file->is_special = false;
+  file->rdev_major = 0;
+  file->rdev_minor = 0;
   return file;
 }