run: tweak logic how we set $TERM for activated services

run: tweak logic how we set $TERM for activated services

There are two cases when we invoke a service on a TTY:

1. We ourselves are connected to a TTY and would intend to enable PTY
   forwarding.
2. We are allocating a TTY but are not ourselves connected to a TTY and
   just want to input/output to pipe or other non-TTY fd.

Let's propagate $TERM only as-is in the first case. In the 2nd case,
let's explicitly set $TERM to "dumb", so that invoked progams do not
issue needless ansi sequences, since we are not propagating them to a
terminal either.

This should be a much safer result, for cases where people include
invocations of systemd-nspawn with full TTY allocation in a shell
pipeline or so.

(of course, the user can always explicitly override this)