Validate passwd.shell.<user> credentials with the same login-shell
rules used for sysusers.d shell fields before writing passwd entries.
Reproducer: CREDENTIALS_DIRECTORY=... systemd-sysusers --root=...
with --inline 'u creduser 999 "Cred User" / -'
Before: relative-shell was written as the login shell and the command
exited successfully.
Follow-up: 99e9f896fb