Missing sFamilyClass in OS/2 table

[wyuenho]

I've recently tried to filter all installed fonts on my mac in objective-c, macOS's CoreText has the ability to filter font descriptors by font family classes such as old style serif, slab serif and so on. When I was investigating why all the DejaVu fonts are grouped as unknown, I've come across the TrueType OS/2 table and the sFamilyClass. It appears AppKit simply relies on the data in this field to return the font family class info. This field is missing in all DejaVu fonts. It would be nice if this can be filled in.