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>

I thought so as well, but I was kind of baffled that the jar didn't start as soon as it is processed with the jar command. Maven does not put the directories into the zip, but jar does and for some reason it puts the dirs behind the files it inserts.
Assume that you add a file org/example/Example.class to the jar with
`jar uvf archive.jar org`
this will put the files into the jar like this:
```
org/example/Example.class
org/
org/example/
```
Since the unzip code has created the dirs already when creating the Example.class file, it will fail when trying to create the org dir.
```
Failed to run fat jar
java.lang.IllegalStateException: Failed to create directory
        at org.vertx.java.platform.impl.FatJarStarter.unzipJar(FatJarStarter.java:279)
        at org.vertx.java.platform.impl.FatJarStarter.unzipIntoTmpDir(FatJarStarter.java:266)
        at org.vertx.java.platform.impl.FatJarStarter.go(FatJarStarter.java:111)
        at org.vertx.java.platform.impl.FatJarStarter.main(FatJarStarter.java:60)
```