prepare-data-dir leaves files with a second name to their owner (closes #143)

A hard link in /data shares its owner with a file that can be anywhere
on the same filesystem, so chowning it gave that file to the netwatch
user. A file with more than one name now keeps its owner and its path
is logged.

Owners are now set on the opened directory, file or link after
checking it, as chmodDir sets modes, so a hard link swapped in
afterwards is caught. Every other file is opened without following it;
a symbolic link is given to the netwatch user itself, and devices,
named pipes and sockets keep their owner. This is built only for
Linux: prepare-data-dir runs only in the Linux image, and macOS has
neither O_PATH nor AT_EMPTY_PATH; elsewhere it returns an error. A
failed owner change names the file.

Model: opus-5-5

go.mod

@@ -17,6 +17,7 @@ require (
 	github.com/slok/go-http-metrics v0.13.0
 	github.com/spf13/viper v1.21.0
 	go.uber.org/fx v1.24.0
+	golang.org/x/sys v0.47.0
 )
 
 require (
@@ -41,7 +42,6 @@ require (
 	go.uber.org/multierr v1.10.0 // indirect
 	go.uber.org/zap v1.26.0 // indirect
 	go.yaml.in/yaml/v3 v3.0.4 // indirect
-	golang.org/x/sys v0.47.0 // indirect
 	golang.org/x/text v0.40.0 // indirect
 	google.golang.org/protobuf v1.36.11 // indirect
 )