Added advanced option for temp filenames prefix

I added an advanced OptString option of FILEPREFIX. This allows the tester to set a custom prefix to the .txt and .bat temporary files. This is useful for post-engagement cleanup, especially when the module creates temporary files but is not able to remove them. The default value for FILEPREFIX is an empty string.

Not much changed, but I still tested the changes against a Win7 Enterprise host. The module created filenames with prefixes as expected. Command execution still worked as expected.