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).

tests/test_chunk.c

@@ -118,11 +118,11 @@ static void test_chunk_dir_entry_roundtrip() {
     dir->is_dir = true;
 
     if (use_metadata) {
-      reg->metadata = file_metadata_create(&st);
+      reg->metadata = file_metadata_create(file_path, &st, false, false);
       EXPECT_NOT_NULL(reg->metadata);
       struct stat dst;
       EXPECT_EQ_INT(stat(dir_path, &dst), 0);
-      dir->metadata = file_metadata_create(&dst);
+      dir->metadata = file_metadata_create(dir_path, &dst, false, false);
       EXPECT_NOT_NULL(dir->metadata);
     }