clarify use of outfilename parameter

This highlights that 'outfilename' is no longer an accurate name for the
parameter, since it can also be a subdirectory ... but we can fix that
later.
fix bug with repeated invocations of "osc service dr"

This source service did not clean up the empty directory/directories
left behind after unpacking the archive.  osc would then transfer them
back into the checked-out package's working directory, where they would
conflict with future runs of "osc service dr".