Permit use of $XDG_RUNTIME_DIR for user files

[jcpunk]

Some user files are basically worthless when the connection ends - such as the agent socket. Other files, like forwarded kerberos tickets, may have value after the session terminates.

GSSAPICleanupCredentials is a good step, but for many systems /tmp is a persistent volume (not tmpfs). Having active, unencrypted login authentication credentials on a remote persistent volume is not great.

https://bugzilla.mindrot.org/show_bug.cgi?id=2950 https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html#variables