On a system with 30 extensions listed by xdpyinfo, truss -c
reports this saves quite a few system calls by batching the
QueryExtension requests instead of a round-trip for each one:

                      Xlib      xcb
writev                  40       11
poll                    80       22
recv                   117       29
total (*)              464      296

(*) total includes all system calls, including many not shown since
their count did not change significantly.   There was one additional
set of open/mmap/close etc. for loading the added libX11-xcb library.

Over a tcp connection, this reduced both the number of packets,
and due to tcp packet header overhead, the overall amount of data:

                      Xlib      xcb
TCP packets             93       35
TCP bytes            11554     7726

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Reviewed-by: Jamey Sharp <jamey@minilop.net>