Fix misc problems with thumbnails on MS-Windows

Fix misc problems with thumbnails on MS-Windows

* lisp/image/image-dired-external.el (image-dired-pngcrush-thumb):
Fix deletion of intermediate file.
(image-dired-cmd-pngcrush-options)
(image-dired-cmd-create-standard-thumbnail-options): Use %u for
file:// URI.
(image-dired--file-URI): New function.
(image-dired-create-thumb-1, image-dired-create-thumb-2)
(image-dired-set-exif-data): Use it to generate correct URI on
MS-Windows.

* src/w32image.c (Fw32image_create_thumbnail): Copy the file names
before mirroring their slashes.