metadata times: -U/--atimes, -N/--crtimes, --open-noatime, -O/-J

Capture+transmit source atime (pre-read stat; O_NOATIME sender guard) and birth
time (statx STATX_BTIME); receiver restores atime with mtime (crtime not settable
portably -> transmitted, explicitly not applied). --open-noatime is client-only.
-O/-J documented as accepted no-ops (FastSync never preserves dir/symlink times).
Wire: metadata frame gains atime/crtime val+sec+nsec; PROTOCOL_VERSION
2.11.0->2.12.0. Review fixes: gate atime capture to Linux (no epoch clobber on
non-Linux), close fd on fdopen failure, honest -O/-J status (Compat no-op).

src/shared/file_send.c

@@ -78,7 +78,7 @@ bool file_send_sendfile_with_skip(File* file, int file_descriptor, bool use_meta
   if (use_metadata && !metadata_send(file_descriptor, file->metadata))
     return false;
 
-  int fd = open(file->path, O_RDONLY);
+  int fd = file_open_for_read(file->path);
   if (fd == -1) {
     log_perror("Could not open file for sendfile");
     return false;