[racket-dev] raco link

I've pushed some changes to `raco link':

 * The `-s'/`--show' flag is now `-l'/`--list'.

   The old flag was closer to `planet show', but after being away from
   `raco link', I keet typing `-l'. Is there a precedent for `-l', or
   should I change it back?

 * The `-r'/`--remove' flag removes in both the user-specific and
   installation-wide links by default. A new `-u'/`--user' flag
   constrains removal to user-specific links.

   The old behavior confused Robby and me. Also, per Robby's
   suggestion, `raco link' in remove mode complains if it finds
   nothing to remove.

 * Combining `-r'/`--remove' with `-n'/`--name' and no directories
   removes all links matching the collection name.

   This change addresses Jay's comment. The new behavior is less
   regular (0 directories => "or"; 1 or more directories => "and"),
   but irregular command-line handling seems to work better if it more
   often does what you meant.

 * The `raco link' command writes relative paths into a collection
   links file.

   This change was Eli's suggestion. Relative paths work better if,
   say, your home directory gets mounted with a varying path. This
   change also fixes the bug that Jay found.

 * The collection links file now supports 'root links that add
   collection roots (i.e., diretories that contain collections, as
   opposed to collection-specific links).  Add/remove such links with
   `raco link --root'.

   This change was also Eli's suggestion. It lets a links file even
   more directly replace "PLTCOLLECTS".