Code maintenance: OS package for file system

- OS custom internal package for file system interaction
- Remove fileManager external dependency
- Closer API to Go's native API on the OS
- Create directories at startup
- Better testability
- Move Unsetenv to os interface

internal/publicip/fs.go

@@ -0,0 +1,27 @@
+package publicip
+
+import "github.com/qdm12/gluetun/internal/os"
+
+func persistPublicIP(openFile os.OpenFileFunc,
+	filepath string, content string, uid, gid int) error {
+	file, err := openFile(
+		filepath,
+		os.O_TRUNC|os.O_WRONLY|os.O_CREATE,
+		0644)
+	if err != nil {
+		return err
+	}
+
+	_, err = file.WriteString(content)
+	if err != nil {
+		_ = file.Close()
+		return err
+	}
+
+	if err := file.Chown(uid, gid); err != nil {
+		_ = file.Close()
+		return err
+	}
+
+	return file.Close()
+}