authutil: save IceAuthFile on XDG_RUNTIME_DIR if set
This is something that should be per user session and not preserved.
In this way we also fix deadlocks on IceLockAuthFile when the -c and -l files have already been saved and not deleted (with a full logout/login, at least).
https://bugs.freedesktop.org/show_bug.cgi?id=106432
Fixes: #6 (closed)