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_receive.h

@@ -10,6 +10,8 @@
 File* file_receive(const Config* config, int file_descriptor);
 File* file_receive_directory(int file_descriptor);
 File* file_receive_hardlink(int file_descriptor);
+File* file_receive_special(int file_descriptor);
+bool file_special_rdev_valid(int32_t major, int32_t minor, mode_t mode);
 File* receive_incremental_check(int fd, const Config* config, bool* skipped);
 
 /* A received delete-manifest frame: the keep-set (`keeps`, destination-relative