sxiv executable smart find

If the sxiv in the default location does not exist, try to find it using some heuristics.
variable with sxiv location renamed
customizable activation key
slash ('/') deduplication in image paths
the new sxiv 'mark' functionality used instead of a kludgy echo
it is possible to use the already installed unpatched sxiv; patch removed

Minor compatibility break! New default key for marking files.
README updated