Add a "-r" option to lns, which causes it to mirror an entire

Add a "-r" option to lns, which causes it to mirror an entire
directory tree in symlinks by constructing a parallel directory
hierarchy and populating it with symlinks to all the files in the
original one. I've wanted a tool that does that for ages, but never
got round to sitting down and writing it; today I suddenly realised
it's almost trivial to add as an extra option in lns, because nearly
all the required infrastructure is already present.

git-svn-id: svn://svn.tartarus.org/sgt/utils@8171 cda61777-01e9-0310-a592-d414129be87e