systemd: Fix relative ln add-wants wrapper

The patch to allow systemd to work with old "ln" versions that don't
support --relative didn't work properly in the the meson-add-wants.sh
script.

This results in all the links in systemd's "*.wants" directories being
broken, e.g.
/usr/lib/systemd/system/multi-user.target.wants/getty.target ->
  ../../../../usr/lib/systemd/system/getty.target
There is one too few ".." in that relative link.

The problem is that the script is called with the link name being either a
file or an existing directory.  In the latter case, ln creates the link in
the directory using the name of the target.  This means the link is one
level deeper than the relative link making code thinks.

The solution used is to only dirname the link, moving up a level, if it's
not a directory, to mimic ln's logic in how it creates links.

Signed-off-by: Trent Piepho <tpiepho@impinj.com>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>