Merge pull request #4067 from poettering/invocation-id

Add an "invocation ID" concept to the service manager

diff --cc TODO
Simple merge

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

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

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

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

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