New headers and parameters from RFC 3455 and UUI drafts

These changes derive from mail by John Gregg (see http://list.resiprocate.org/archive/resiprocate-devel/msg08665.html). I added additional header to exchange User-to-User information via SIP.
User-to-User would be good to have.  Some people already use ExtensionHeader to access User-to-User headers in messages.

Unfortunately, the ExtensionHeader constructor asserts() if somebody tries to use it for a header name that is known to the stack.  So after we accept this change, anybody with ExtensionHeader uui("User-to-User") is going to experience a crash.  It will probably not be a good idea to backport this change to the 1.9.x series, let people encounter it in 1.10.x.  Maybe also improve the ExtensionHeader construct so it throws an Exception instead of assert()