systemd: add compatibility symlink for 'shutdown'

In addition to the 'halt', 'poweroff', 'reboot' symlinks pointing to
systemctl, add 'shutdown'.

Signed-off-by: Anssi Hannula <anssi.hannula@bitwise.fi>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>