Improve thumbnail generation on MS-Windows

Improve thumbnail generation on MS-Windows

* lisp/image/image-dired-external.el (image-dired-create-thumb-2):
Optimize PNG thumbnails.
(image-dired-pngnq-thumb, image-dired-pngcrush-thumb)
(image-dired-optipng-thumb): On MS-Windows, invoke the PNG
optimization programs synchronously.
(image-dired-cmd-create-thumbnail-program)
(image-dired-cmd-create-thumbnail-options)
(image-dired-cmd-pngcrush-program)
(image-dired-cmd-optipng-program)
(image-dired-cmd-create-standard-thumbnail-options)
(image-dired-cmd-rotate-original-program)
(image-dired-temp-rotate-image-file)
(image-dired-cmd-write-exif-data-program)
(image-dired-cmd-write-exif-data-options): Doc fixes.
(image-dired-thumb-queue-run): Don't allow more than 30 concurrent
thumbnail-creation jobs on MS-Windows.