Use verity sharing for user services and nspawn too (#39313)

https://github.com/systemd/systemd/pull/39168 made verity sharing
opt-in, and enabled it for system services.
Also enable it for user services for RootImage/etc, and for nspawn, for
the same reasons.

diff --cc src/mountfsd/mountwork.c
Simple merge

diff --cc src/shared/dissect-image.c
Simple merge