logind: log peer ID when shutdown is called

logind: log peer ID when shutdown is called

The io.systemd.Manager.{PowerOff,SoftReboot,Halt,Kexec} manager
varlink and bus methods log the peer ID when calling shutdown.

The logind code is missing this, so this commit adds a similar
logging now. The code is quite similar to the one in existing in
src/core/manager.c but its hard to share code so this adds a bit
of duplication.