expiry changes

The P3P specification working group has adopted some
changes related to the EXPIRY element. New text for section
2.3.2.3 is attached. These changes eliminate the use of
HTTP headers for determining policy and policy reference
file expiry, move the EXPIRY element to be a child of
the POLICIES element, and clarify the meaning of expiry.

In addition, we will change sections 3.2.1 and 3.2.2 to
move the EXPIRY element to be a child of the POLICIES
element instead of the POLICY element (and update
the DTD and schema accordingly). 

The paragraph before example 2.3 in
section 2.3.2.7 will be changed to say:

 "The policy that applies to a cookie applies until the policy 
expires, even if the associated policy reference file expires
prior to policy expiry (but after the cookie was set). 
If the policy associated with a cookie has expired, then the user agent 
SHOULD reevaluate the cookie policy before sending the cookie. 
In addition, user agents MUST use only non-expired policies 
and policy reference files when evaluating new set-cookie events."