KWSys: Do not mangle UNC paths in ConvertToUnixOutputPath (#10206)

This method replaces '//' with '/' to make the paths look nicer.
Originally it correctly skipped a leading '//' in a UNC path as the
comment says.  However, commit "Removed extra variable initializations"
(2005-04-15) accidentally removed the "pos=1" initializer.  It was then
incorrectly restored by commit "Added missing variable initialization"
(2005-04-15) as just "pos=0".  Restore the proper initializer.

The test for this added by commit "better coverage" (2006-07-31)
included incorrect output for a sample UNC-like path.  Fix it.