systemctl: allow to change the default target without the --force switch

systemctl: allow to change the default target without the --force switch

Currently "systemctl set-default" will fail to change the default target
due to the 'default.target' being a symlink which is always the case.

To work around this, the user must specify the "--force" switch to be
able to overwrite the existing symlink.

This is clearly a regression that was introduced by commit  718db96199e
since it worked before without the "--force" switch and the man pages do
not mention that you need to specify it. It is expected that this is a
symlink.

So just explicity set the force flag to make it work again.

https://bugs.freedesktop.org/show_bug.cgi?id=76623

Reported-by: <code@progandy.de>