BZ449637:  when a farjar is run with a java agent, it unzips the wrong j...

...ar giving a NPE

retrieve jar name based on the class FatJarStarter instead of the class loader URL list

(this commit also changes the temp dir creation to use File instead of String operations
and fixes an issue where the jar couldn't be extracted if it was manipulated with jar
command, which apparently puts the dirs after the files on updates)

Signed-off-by: Alex <alexlehm@gmail.com>

Please test on both Linux and Windows :) We had issues in the past with OS dependent bugs in URL handling logic because fixes hadn't been tested on both OSes.