core/service: only pass socket fds to control processes

core/service: only pass socket fds to control processes

If socket is used as stdio, we'd currently imply EXEC_PASS_FDS
and dump the whole set of fds to the control processes. This is
pretty much unexpected and unnecessary though, instead let's
pass only the socket fds.

Yes, this is a compat break, but a relatively minor one I'd
argue. And we can always revisit things if users do complain.