systemctl: make sure we terminate the bus connection first, and then close the pager (#3550)

If "systemctl -H" is used, let's make sure we first terminate the bus
connection, and only then close the pager. If done in this order ssh will get
an EOF on stdin (as we speak D-Bus through ssh's stdin/stdout), and then
terminate. This makes sure the standard error we were invoked on is released by
ssh, and only that makes sure we don't deadlock on the pager which waits for
all clients closing its input pipe.

(Similar fixes for the various other xyzctl tools that support both pagers and
-H)

Fixes: #3543

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