Fix for bug #3727: Updating image node requires new image upload

Fix for bug #3652: Cannot upload gif files. Disable GIF image upload if image library has no GIF creation support.
Partial fix for #3423: Module should not output content directly. Removed most of echo(), die() and print() functions.
Fix for bug #2484: SQL error when creating image in preview screen (if no term is selected, "personal:"=disabled).
Modified image_cron() to clean up old temporarary image AND thumb files.
Fixed image_validate() make effective upload file checking: attempt to upload a file bigger than configured max size is now reported, attempt to upload a non image file is reported.
Added better checking of (GD) image library capabilities during upload process and thumbnail creation.
Improved image module status report in administer>>images.
Cleaned up _image_make_thumbnail() function to remove duplicate code.
Improved slow and fast image upload so that only supported images formats files are processed.