Parses a header whose content is a list of items with optional "qvalue"s (eg, Accept, Accept-Charset, Accept-Encoding, Accept-Language, TE).
If unacceptable is not null, then on return, it will contain the items with qvalue 0. Either way, those items
will be removed from the main list.
| header |
a header value |
| unacceptable |
on return, will contain a list of unacceptable values |
|
a SList of acceptable values (as allocated strings), highest-qvalue first. |