Windows legacy: fix handling of UNC names

[BoboTiG]

The legacy implementation was not handling UNC names properly:

Traceback (most recent call last):
  File "check.py", line 6, in <module>
    send2trash(str(file))
  File "\...\plat_win_legacy.py", line 79, in send2trash
    paths = [get_short_path_name(path) for path in paths]
  File "\...\plat_win_legacy.py", line 79, in <listcomp>
    paths = [get_short_path_name(path) for path in paths]
  File "\...\plat_win_legacy.py", line 62, in get_short_path_name
    raise WindowsError(err_no, FormatError(err_no), long_name[4:])
OSError: [Errno 123] La syntaxe du nom de fichier, de répertoire ou de volume est incorrecte.: '\\\\SERVER\\folder\\file.txt'