logind: refuse operations if the target unit is masked or unavailable

logind: refuse operations if the target unit is masked or unavailable

If hibernate.target is masked, and systemctl hibernate is invoked, havoc ensues.
logind starts the hibernation operation, but then doesn't go through with it;
gnome-shell segfaults. Let's be nice to the user and refuse doing anything in
that case.

$ sudo systemctl mask hibernate.target
$ busctl call org.freedesktop.login1 /org/freedesktop/login1 org.freedesktop.login1.Manager CanHibernate
s "no"
$ sudo systemctl hibernate
Failed to hibernate system via logind: Access denied
Failed to start hibernate.target: Unit hibernate.target is masked.

https://bugzilla.redhat.com/show_bug.cgi?id=1468003#c4