see #7275 - allow script to handle restart

When JOSM was started with parameter -Djosm.restart=true, a
user request for restart will simply terminate the application
with exit code 9.
The start script is then responsible for the restart.


git-svn-id: http://josm.openstreetmap.de/svn/trunk@6125 0c6e7542-c601-0410-84e7-c038aed88b3b