io.files.windows: replace open-existing with open-r/w

This fixes error throwing in case of CreateFile failure, and calls
add-completion for the file handle on success.

basis/io/files/info/windows/windows.factor

@@ -232,7 +232,7 @@ M: windows file-systems ( -- array )
     ! timestamp order: creation access write
     [
         [
-            normalize-path open-existing &dispose handle>>
+            normalize-path open-r/w &dispose handle>>
         ] 3dip (set-file-times)
     ] with-destructors ;
 

basis/io/files/windows/windows.factor

@@ -245,14 +245,6 @@ M: windows init-stdio
 : (open-append) ( path -- win32-file )
     GENERIC_WRITE OPEN_ALWAYS 0 open-file ;
 
-: open-existing ( path -- win32-file )
-    flags{ GENERIC_READ GENERIC_WRITE }
-    share-mode
-    f
-    OPEN_EXISTING
-    FILE_FLAG_BACKUP_SEMANTICS
-    f CreateFileW dup win32-error=0/f <win32-file> ;
-
 : maybe-create-file ( path -- win32-file ? )
     ! return true if file was just created
     flags{ GENERIC_READ GENERIC_WRITE }