RFC 6265bis: Update ABNF to define field values

[chlily1]

This adjusts the ABNF for Set-Cookie and Cookie to define field values, not the whole field line, addressing #1302.

[chlily1]

@reschke @englehardt PTAL.

I decided to just refer to set-cookie-string and cookie-string (even though the extra layer of indirection is a bit redundant) because there are still references to those throughout the text and it seemed simpler to keep those.

[reschke]

Works for me, but I would prefer to get rid of essentially unused ABNF productions.

I also note that the paragraph above ("Syntax") needs work; it should only talk about the syntax of the field value, and not discuss ": etc.

[chlily1]

I also note that the paragraph above ("Syntax") needs work; it should only talk about the syntax of the field value, and not discuss ": etc.

Thanks for pointing that out! I've updated that as well.