core: also issue OSC 3008 from service context

core: also issue OSC 3008 from service context

(Note: we also change TEST-13-NSPAWN.machined.sh minimally here, because
it checks for byte precise output of a pty allocated for a service
invocation - which it's not going to get if it claims that the pty is an
all-powerful one. After all this PR ensures that we'll generate the new
OSC sequence on non-dumb terminals associated with services. Hence, set
TERM=dumb explicitly to ensure no ANSI sequences are generated, ever.
Which is a nice test btw that TERM=dumb really does its thing here.)