manager: reexecute on SIGRTMIN+25, user instances only

Before this patch, there was no way to request all running user instances for
reexecuting. However this can be useful especially during package updates
otherwise user instances are never updated and keep running a potentially very
old version of the binaries.

Now assuming that we have enough priviledge, it's possible to request
reexecution of all user instances:

  systemctl kill --signal=SIGRTMIN+25 "user@*.service"

Note that this request is obviously asynchronous as it relies on a
signal. Keeping "systemctl kill" as the only interface should be good enough to
make this obvious and that's the reason why another interface, such as
"systemctl --global daemon-reexec" has not been considered.

PID1 already uses SIGTERM for reexecuting hence sending it SIGRTMIN+25 is a
nop.