Support displays with IPv6 addresses or hosts which resolve to IPv6 addresses

Support displays with IPv6 addresses or hosts which resolve to IPv6 addresses

xcb_parse_display already correctly handled IPv6 displays.  Now, _xcb_open_tcp
uses getaddrinfo, and correctly connects to IPv6 displays.  Displays can use
bare IPv6 addresses, square-bracketed IPv6 addresses, or hostnames which
resolve to IPv6 addresses.

Since IPv6 addresses may include colons, including at the end, revise the
DECnet display parsing code to avoid triggering on IPv6 addresses.

Authorization may not work with IPv6 connections yet.

This commit brought to you by the (display) number ::1:1.1, the letter X,
the Gobby collaborative editor, Josh Triplett, and Jamey Sharp.