[Xcb] xcb-proto-1.7 broke existing xcbgen API

[Jeremy Huddleston]

Back in late 2010(1), someone reported that xpyb failed to build with recent xcb-proto changes.  It seems this regression in xcb-proto was never addressed and unfortunately xcb-proto-1.7 shipped.

I bisected the issue to 76ca2c0b1 which changed xcbgen's API from using "parent" to using "parents".  The patch actually made two completely unrelated changes simultaneously; in the future, please remember to keep one change per patch.

So how should this be addressed?  
  1) We revert the entire thing and then push just the spec changes
  2) We just "flag day" this and move on, changing xpyb.
  3) We support both "parent" and "parents" for maximum compatability?

For 3)