Provide an API to set and reset the System Module Loader

The System Property "system.module.loader" allows the specification of a class name to be used as the System Module Loader. This works to a certain degree, but since the instance is stored in a static final SystemModuleLoaderHolder.INSTANCE variable, it can only be initialized once. This is a problem for the unit tests in JBoss OSGi as they initialize the system repeatedly (e.g. to test various initialization options). This can probably be worked around by putting a holder in there instead of the real instance, but that's a workaround...

Another potential problem with the system property approach is that the system module loader instance has to be created by JBoss Modules using a noarg constructor, which limits the initialization options. Both issues could be resolved by providing an API to set and reset the System Module Loade