add X11R6 lndir re-implementatation based on Mike Partin's version

This version includes py.test tmpdir unittests.  It also tries to match
the original X11R6 lndir functionality.  If the src is a symlink then
just re-link to the original.  Relative src paths are interpreted from
the destination, not the cwd.

Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>