Overhaul of client-side XDM-AUTHORIZATION-1:

Overhaul of client-side XDM-AUTHORIZATION-1:
* Make sk_getxdmdata() return an arbitrary string rather than two integers.
  This better matches the spec, even if the current version always returns
  six bytes
* On Unix, for PF_UNIX sockets, return a counter rather than a constant along
  with the PID.  This should allow multiple clients to connect within one
  second, and is what Xlib does.
* On Unix, interpret AF_INET6 addresses like Xlib does, returning the
  embedded IPv4 address for v4-mapped addresses, and six bytes of zeroes
  otherwise.  The latter is silly, but if I'm going to do anything more sane
  I need to check that X servers won't reject it.

[originally from svn r5219]