Fixed the sxiv workaround to handle spaces and be more POSIX compliant.

The previous sxiv scripts would not allow sxiv to browse files if selected file contained spaces. This is  because of the missing doubequotes around the abspath argument.

Functions in Bourne Shell are declared like this:
myFunctionname () {
...
}
There is no 'function' keyword, that's a bashism.

I added a check on script arguments to display a "Usage:" line. This should help when the script is used interactively.

I added some comments on the implementation. It is slow but I do not see another way to do this while conforming to POSIX. This script has very poor performances indeed. listfiles is called two times, which mean the 'find' and the 'sort' get performed two times. This is not acceptable for big folders.  I bet a python script would be much better.