[IZPACK-1173] Make single-instance locking of the installer configurable

This pull requests implements https://jira.codehaus.org/browse/IZPACK-1173:

There is a built-in feature of popping up a message which asks the user whether an already running instance of the same installer should be ignored or rather whether to abort (see attachment).

This can be inconvenient for some use cases (clustering, sub-calls when an installer calls itself with different options,...).

Allow to disable this behavior, the default of enabling it should be unchanged.