Merge pull request #9783 from poettering/get-user-creds-flags

beef up get_user_creds() a bit and other improvements

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

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

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

diff --cc src/login/loginctl.c
Simple merge

diff --cc src/login/logind-core.c
Simple merge

diff --cc src/mount/mount-tool.c
Simple merge

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

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

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

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