Bug#685198: Please let me set a site-wide default of TempDir=$XDG_RUNTIME_DIR!

[Trent W. Buck]

As at Jessie, (audacity=2.0.6-2), the default directory is /var/tmp/audacity-<USERNAME>.

This can't be changed in the environment (e.g. export AUDACITY_TMPDIR=$XDG_RUNTIME_DIR).
This can't be changed (for all users) in /etc or /usr (e.g. /usr/share/audacity/audacity.cfg).
This can only be fixed in each user's $HOME/.audacity-data/audacity.cfg.

Per http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html,
please use $XDG_RUNTIME_DIR as the default.

Typically XDG_RUNTIME_DIR is auto-managed by systemd-logind,
such that it is stored in RAM and has available space equal to 50% of total RAM.

The manpage indicates that the temp dir should be "fast";
RAM is *MUCH* faster than a local disk :-)

The manpage says it defaults to /var/tmp instead of /tmp so that if
the entire OS crashes, the temporary files can be recovered.
Since my OS doesn't crash, I don't need to worry about that :-)

The relevant code is src/AudacityApp.cpp:1090:AudacityApp::OnInit().

Please let me set a site-wide default of TempDir=$XDG_RUNTIME_DIR!