bus-util: rename bus_open_transport() to bus_connect_transport()

bus-util: rename bus_open_transport() to bus_connect_transport()

In sd-bus, the sd_bus_open_xyz() family of calls allocates a new bus,
while sd_bus_default_xyz() family tries to reuse the thread's default
bus. bus_open_transport() sometimes internally uses the former,
sometimes the latter family, but suggests it only calls the former via
its name. Hence, let's avoid this confusion, and generically rename the
call to bus_connect_transport().

Similar for all related calls.

And while we are at it, also change cgls + cgtop to do direct systemd
connections where possible, since all they do is talk to systemd itself.