GBytes makes more sense, because it's immutable.

Also, since at other places we use GBytes, having
different types is combersome and requires needless
conversions.

Also:

- avoid nm_utils_escape_ssid() instead of _nm_utils_ssid_to_string().
  We use nm_utils_escape_ssid() when we want to log the SSID. However, it
  does not escape newlines, which is bad.

- also no longer use nm_utils_same_ssid(). Since it no longer
  treated trailing NUL special, it is not different from
  g_bytes_equal().

- also, don't use nm_utils_ssid_to_utf8() for logging anymore.
  For logging, _nm_utils_ssid_escape_utf8safe() is better because
  it is loss-less escaping which can be unambigously reverted.