xrootd/cmd/xrd-ls: change empty lines in printout
xrootd/cmd/xrd-ls: change empty lines in printout

Old behaviour:
 * Print empty line after any directory listing.
   - This means that the full printout ends with an empty line
New behaviour:
 * Print one empty line between requested directories.
   - This means that no empty line is printed after the last of multiple
     directories.
 * Print one empty line before printing a subdirectory with -R
   - This means an empty line occurs only after listing a directory in
     the recursive mode when there are actually subdirectories.

go-hep/hep,go-hep/hep