Automatically find default JDK on Mac OS X and GNU/Linux.

On Mac OS X and GNU/Linux the default JDK is probed and, when found,
used as the default jde-jdk-registry. If JAVA_HOME is not set then
jde-jdk will be set to this found JDK. Also updated default
jde-help-docsets to latest Oracle sites and associated docs.

git-svn-id: https://jdee.svn.sourceforge.net/svnroot/jdee/trunk@285 381a1f10-3442-0410-997e-ef331cd04102