Pursue the creation of XDG_SESSION_TMPDIR specification

[andersk]

Lots of applications try to create sockets in $HOME, which fails in AFS, for lack of a better place to put them. davidben found  a proposed XDG specification that would solve this problem by providing an $XDG_SESSION_TMPDIR directory for session-local state (like our $ATHENA_SESSION_TMPDIR). We should try to work with the XDG people to get it adopted and used by these applications.