core: automatically add dependency on systemd-remount-fs.service if StateDirectory...

core: automatically add dependency on systemd-remount-fs.service if StateDirectory= is used

And similar for other settings that require a writable /var/.

Rationale: if these options are used for early-boot services (such as
systemd-pstore.service) we need /var/ writable. And if /var/ is on the
root fs, then systemd-remount-fs.service is the service that ensures
that /var/ is writable.

This allows us to remove explicit deps in services such as
systemd-pstore.service.