Fix initialisation of username on Windows (rebased onto develop)



This is the same as gh-539 but rebased onto develop.

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This is an intermittent fix for https://trac.openmicroscopy.org.uk/ome/ticket/9967. It make ``bin\omero admin start`` pick up the ``omero.windows.user`` setting properly and allows for auto-setting the Log On As user of the OMERO.master service.