tree-wide: use device_get_sysattr_safe_string()

tree-wide: use device_get_sysattr_safe_string()

The obtained strings are passed to another function, e,g, handled as a
path and opened, printed to the terminal, written to a file, saved to
udev database as udev property, exposed through DBus, passed to logger,
and so on. Hence, these should not contain any malicious characters.