Previously if send-display-update was set to false we would also
remove UPDATE from our Allow: headers.  This is unnecessary.  The
UPDATE message is useful in SIP transactions even if we're not sending
display updates.

With this commit, we add a new boolean profile flag, allow-update.  If
set to true we'll send Allow: UPDATE.  If set to false, we will not.
If there is a conflict with another setting that requires UPDATE
support, the allow-update parameter will win and a warning will be
printed.

ref: RFC 3311