Rewrite the Set-Cookie header parser entirely.

The new parser is almost RFC 6265 compliant as the previous
implementation but has some extensions:

- It can parse double-quoted values with unsafe characters inside
  escaped with the backslash.

- It parses a date value of the expires attribute in the way the RFC
  describes, with an exception that it allows omission of the seconds
  field.  Some of the broken date representations that used to pass
  are now treated as error and ignored.

- It can parse a Set-Cookie value that contains multiple cookie
  definitions separated by comma, and commas put inside double quotes
  are not mistaken as definition separator.