fix #8697 - Disable restart action with JVMs that do not provide `sun.java.command` system property (ex: JDK 6u24)

git-svn-id: http://josm.openstreetmap.de/svn/trunk@5951 0c6e7542-c601-0410-84e7-c038aed88b3b
Merge branch 'master' into mirror