[Merge] lp:~xnox/upstart/user-log-dir into lp:upstart

[Dmitrijs Ledkovs]

XDG_RUNTIME_DIR guarantees that we get the same dir no matter how many times we login.
We can manage multiple pids/sockets ourself per user.

The file we store there will be pid e.g. $XDG_RUNTIME_DIR/upstart/12345
For another session (for the same user) we will have pid e.g. $XDG_RUNTIME_DIR/upstart/15001
This is to support one upstart session per login session.

The will know the current session via environment variable.
Also we can implement `initctl list-sessions` command simply by iterating $XDG_RUNTIME_DIR/upstart/.

XDG_CACHE_HOME is not guaranteed to be cleared, while XDG_RUNTIME_DIR will always be fresh after reboots & complete logouts.
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https://code.launchpad.net/~xnox/upstart/user-log-dir/+merge/143091
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