Merge pull request #18971 from poettering/sysusers-creds

let's read LoadCredentials=/SetCredentials= style cred in sysusers/firstboot and when asking for passwords

diff --cc TODO
Simple merge

diff --cc src/core/execute.c
Simple merge

diff --cc src/core/load-fragment.c
Simple merge

diff --cc src/firstboot/firstboot.c
Simple merge

diff --cc src/nspawn/nspawn.c
Simple merge

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

diff --cc src/sysusers/sysusers.c
Simple merge