Help users with incorrect / permission bits (#41431)

Help users with incorrect / permission bits (#41431)

This error causes the computer to pass the emergency.target and go to
graphical.target.
Then, your window manager will have problems because it cant access any
directories, your network manager wont startup the network. In my case,
the screen just goes black. Ideally, you'd get an error message
explaining this edge scenario that's occuring to you, and an emergency
shell that makes it easy to run the necessary chmod 0755 / to proceed
with booting. IDK not sure if this is the correct way to implement this,
sorry it's my first contribution.

I ran
`meson test -C build`
and got

Ok:                1806
Fail:              26
Skipped:           25
on my cloned systemd repo before any changes, and got the same result
after my commit ¯\_(ツ)_/¯
So I hope I did that right.
Thanks