add icing for DestroyNotify, (Un)GrabServer, GetSelectionOwner
Ignore non-<item> elements in current xcb-proto (Thanks mountains)
Use UDP with getsockopt(SO_PEERCRED) instead of TCP

Thanks to
http://welz.org.za/notes/on-peer-cred.html
I learned about SO_PEERCRED, which is a much cleaner way of doing what
we were doing before with endpoint_cmdline().