link2symlink: don't bug when the same file is linked several times.

Before:

    $ rm -f foo bar baz
    $ echo TEST > foo
    $  ./src/proot ln foo bar
    $  ./src/proot ln foo baz
    $ ls -l foo bar baz
    lrwxrwxrwx 1 cedric users 35 Oct 16 21:47 bar -> /home/user/foo.l2s
    lrwxrwxrwx 1 cedric users 35 Oct 16 21:47 baz -> /home/user/foo.l2s
    lrwxrwxrwx 1 cedric users 35 Oct 16 21:47 foo -> /home/user/foo.l2s
    $ ls -l /home/user/foo.l2s
    lrwxrwxrwx 1 cedric users 35 Oct 16 21:48 /home/user/foo.l2s -> /home/user/foo.l2s

Now:

    $ rm -f foo bar baz
    $ echo TEST > foo
    $  ./src/proot ln foo bar
    $  ./src/proot ln foo baz
    $ ls -l foo bar baz
    lrwxrwxrwx 1 cedric users 35 Oct 16 21:47 bar -> /home/user/foo.l2s
    lrwxrwxrwx 1 cedric users 35 Oct 16 21:47 baz -> /home/user/foo.l2s
    lrwxrwxrwx 1 cedric users 35 Oct 16 21:47 foo -> /home/user/foo.l2s
    $ ls -l /home/user/foo.l2s
    -rw-r--r-- 1 cedric users 5 Oct 16 21:49 /home/user/foo.l2s