[PATCH xorg] Support multi-device pointer barriers

[Jasper St. Pierre]

This implements the currently unimplemented portion of the XFixes
barriers specification, allowing clients to specify a set of devices
for which the barrier is active. This should be in accordance with
the XFixes specification, so if it isn't, please let me know.

( Ignore the previous set of patches; I had a bad rebase, but forgot
  to `make install` before re-running tests. Still trying to get a
  hold of git-send-email, as well. Sorry for all the noise. )