All helper client code now uses xcb, so calling XSetAuthorization() is
no longer needed.

This is the last reference to libX11 from helper clients, so linking
with x11-xcb and libX11 is no longer required.

Also drop (unneeded?) linking with libXau.

Also drop installing these prerequistes on AppvVeyor.

Also move prototypes for functions in winauth.c from win.h into a new
header, winauth.h, and include that where needed.