XCBSync is a trivial utility function that should be used exceedingly rarely and
that has API warts.

It's trivial: It simply issues a GetInputFocus request, then waits for and
ignores the reply.

It should be used rarely because the sync operation forces a round-trip, which
is expensive and almost never necessary. Many programs over-use this operation;
one possible cause is that the round-trip delay can hide initialization
problems, which can make incorrect code appear to work.

It has API warts. The caller must pass in an XCBGenericError** (possibly NULL)
to be filled in if an X error occurred in response to the GetInputFocus request,
but the reference implementation can't produce any error for this request as far
as I can tell, and the extra argument is confusing for programmers. XCBSync also
returns a boolean indicating whether the sync was successful, which will only be
false if an X error occurred (as above, can't happen) or if the connection has
failed. But nobody checks this return value anyway, and even if a connection
failure error is ignored it will be detected on some later request.

That's three strikes: let's get rid of XCBSync. A similar function could go into
XCBAux.