Default named instance should use subfamilyNameID 2 or 17

The [spec](https://docs.microsoft.com/en-us/typography/opentype/spec/fvar#instancerecord) says:

> If an instance record is included for the default instance (that is, an instance record has coordinates set to default values), then the nameID value should be set to either 2 or 17, and the postScriptNameID value should be set to 6.

However, fontmake assigns a new name ID, even if the subfamilyNameID *value* matches that of 2 or 17.

FWIW, I think [the spec is being too picky](https://github.com/MicrosoftDocs/typography-issues/issues/946), but fontbakery is now [failing this case](https://github.com/googlefonts/fontbakery/issues/3708).