path_auditor: check filenames for basic platform validity (issue2755) Example (on Windows): $ hg parents $ hg manifest tip con.xml $ hg update abort: filename contains 'con', which is reserved on Windows: con.xml Before this patch, update produced (as explained in issue2755): $ hg update abort: No usable temporary filename found I've added the new function checkwinfilename to util.py and not to windows.py, so that we can later call it when running on posix platforms too, for when we decide to implement a (configurable) warning message on 'hg add'. As per this patch, checkwinfilename is currently only used when running on Windwows. path_auditor calls checkosfilename, which is a NOP on posix and an alias for checkwinfilename on Windows.