add P3P headers to endpoints IE might care about

Reopened. @shane-tomlinson or @lloyd, mind taking a look?


I split this off from the main blob of third-party cookie discussion.

Add P3P headers to IE family browsers (a little searching suggests IE9 and, presumably, IE10 also care about this header).

Do I need to add tests to confirm this header is appended if the user-agent header contains an IE-like string?