We usually don't care, but here the existence of socket
is public API to a certain degree and signals availability
of the service (userdbd in particular, oomd is checked in
core-varlink.c). Hence let's be more careful and remove them
if stopped.
It's lightweight and generally useful, so it should be enabled by default. But
users might want to disable it for whatever reason, and things should be fine
without it, so let's make it installable so it can be disabled if wanted.
Fixes#15175.