Session management is a bit awkward right now in that the list of active
sessions is managed by Client, resulting in operations like closing a
session being split between several functions in Client and Session.
This patch moves all session management to the Session class. Closing a
session is now entirely in Session::close().
This will make managing a separate HTTP session list a bit simpler.
Lots of editorial spec bugs here, but these changes largely affect how
the unhandledPromptBehavior capability is handled. We also now set an
additional capability for the default User Agent string.
