Rewritten EMBEDEXTERNALIMAGES support
Rewritten EMBEDEXTERNALIMAGES support

Added two new functions external_image_exists & get_external_image and moved EMBEDEXTERNALIMAGES code from filesystem_image_exists & get_filesystem_image.
Added defines EXTERNALIMAGE_MAXAGE, EXTERNALIMAGE_TIMEOUT & EXTERNALIMAGE_MAXSIZE.
Added EMBEDEXTERNALIMAGES code to find_html_images for more specific external image matching allowing for query strings and path info.
mariez/phplist3,phpList/phplist3,bramley/phplist3,danwaterloo/phplist3,mariez/phplist3,mariez/phplist3,phpList/phplist3,phpList/phplist3,samtuke/phplist3,danwaterloo/phplist3,danwaterloo/phplist3,samtuke/phplist3,tarekdj/phplist3,bramley/phplist3,samtuke/phplist3,tarekdj/phplist3,phpList/phplist3,bramley/phplist3,danwaterloo/phplist3,phpList/phplist3,tarekdj/phplist3,samtuke/phplist3,tarekdj/phplist3,bramley/phplist3,tarekdj/phplist3,bramley/phplist3,mariez/phplist3,danwaterloo/phplist3,mariez/phplist3,samtuke/phplist3