Refactor auth code to get display number from xcb_connect

Change xcb_connect to pass the display number to _xcb_get_auth_info, which
passes it to get_authptr.  This allows get_authptr to stop hacking the display
number out of the sockaddrs of various address families, such as
port - X_TCP_PORT, or the number after the last X in the UNIX socket path. This
also removes a portability bug introduced during the IPv6 changes: relying on
'\0'-termination of the UNIX socket path in a sockaddr_un.

Commit by Jamey Sharp and Josh Triplett.