Moving to Alt-Svc-Used

For #502.

@reschke can you review this.  Please merge it if you like it.
On 2014-06-06 20:03, Martin Thomson wrote:
> In draft-ietf-httpbis-alt-svc.xml:
>
>>  </t>
>>  <figure>
>>  <artwork type="abnf2616">
>> -Service = uri-host [ ":" port ]
>> +Alt-Svc-Used     = ("1" / "0") 0*( SP Alt-Svc-Used-Ext )
>
> semicolons, ack. Do I need OWSP or something like that?

Alt-Svc-Used = ("1" / "0") ( OWS ";" OWS Alt-Svc-Used-Ext)

(and we need a sentence stating what it's for)