Option -s added

Option -s adds a literal prefix (this is without a /) to the source
path.  It only does this on the fly when accessing the source file, so
it has no effect on -p etc. options.

This new option is mainly for use with option -l, for example see
"dirlist" http://permalink.de/tino/dirlist which only outputs names,
not pathes.  This way you can add the missing path again without
complex "cd" constructs.
