systemctl: delay pager/polkit agent opening as much as possible

systemctl: delay pager/polkit agent opening as much as possible

In https://github.com/systemd/systemd/issues/3543, we would open the pager
before starting ssh, and the pipe fd was "leaked" into the ssh child as the
stderr fd. Previous commit fixes bus-socket to nullify stderr before launching
the child, but it seems reasonable to also delay starting the pager.
If we are going to croak when trying to open the transport, it seems better
to do this before starting the pager.

This commit would also fix #3543 on its own.