On Windows, lock the opened mutex file to insure *nix sopen() will fail

Even though we use O_EXCL, networked file systems still sometimes allow the
secondary open() to succeed. We use record locking in xpdev's sopen()
implementation for *nix, so this lock will insure such opens (and locks) will
now fail.

This is an attempt to address occassional error on Vertrauen:
ERROR 2 (No such file or directory) renaming *.rep to *.bad