Run an installer with privileged permissions

In some cases, an installation has to be performed with administrative permissions.

There is currently no mechanism to ask the JVM to run with such elevated / privileged permissions. This forces end-users to explicitly take care of running their installers with the right user account.

The intent of this new feature is to (optionally) allow an installer to relaunch itself with elevated permissions. This requires some platform-specific techniques.

The initial prototyping is available from Git: http://github.com/jponge/izpack/tree/exec-admin

In the absence of objections I will merge those changes back to our IzPack Subversion repository for inclusion in 4.2.