Support authentication for IPv6 connections
Support authentication for IPv6 connections

Support AF_INET6 in get_authptr, and refactor to use common code for IPv4 and
v4-mapped IPv6 addresses.

Commit by Jamey Sharp and Josh Triplett.

topcat/xcb-win32,OpenInkpot-archive/iplinux-libxcb,topcat/xcb-win32,topcat/xcb-win32,OpenInkpot-archive/iplinux-libxcb,stapelberg/libxcb,topcat/xcb-win32,topcat/xcb-win32,OpenInkpot-archive/iplinux-libxcb,stapelberg/libxcb,stapelberg/libxcb