Default to a file in /tmp for the restart state.

Default to a file in /tmp for the restart state.

The file is now created in /tmp using the process PID and the
username of the user running i3. The restart state file is only
loaded when restarting (the --restart option is appended to the
command line prior to the restart). That means that renaming the
old state file with the ".old" extension is no longer needed.

This "--restart" switch is supposed to be only used by i3. The
"-L" switch can be used to load a layout (and not delete it
afterwards). We unlink the state file after we load it so that
we don't keep cruft in /tmp or try to restart from an old config
file if restart_state is set.