run/mount/systemctl: don't fork off PolicyKit/ask-pw agent when in --user mode
When we are in --user mode there's no point in doing PolicyKit/ask-pw
because both of these systems are only used by system-level services.
Let's disable the two agents for that automaticlly hence.
Prompted by: #20576
(cherry picked from commit 966f3a24)
(cherry picked from commit fb999b91)