Note that various components (NMFirewallManager, NMAuthManager,
NMConnectivity, etc.pp) all request their own GDBusConnection from
glib's G_BUS_TYPE_SYSTEM singleton.

In the future, let them instead use the D-Bus connection that
NMDBusManager already has.

- NMDBusManager also uses g_bus_get(G_BUS_TYPE_SYSTEM), so in practice this
  is just the same GDBusConnection instance.

- if it would not be the same GDBusConnection instance, it would
  be more correct/logical to use the one that NMDBusManager uses.

- NMDBusManager already aquired the GDBusConnection synchronously
  and it's ready for use. On the other hand, g_bus_get()/g_bus_get_sync()
  has the notion that getting the singleton cannot be done without
  waiting/blocking. So at least it involves locking or even dispatching
  the async reply on D-Bus.

- in "configure-and-quit=initrd" we really don't have D-Bus available.
  NMDBusManager should control whether the other components use D-Bus
  or not. For example, NMFirewallManager should not ask glib for a
  G_BUS_TYPE_SYSTEM singleton only to later find out that it doesn't work.

So if these components would reuse NMDBusManager's GDBusConnection,
then it must have the connection also in regular "configure-and-quit=true"
mode. In this case, we are in late boot and want do connectivity
checking and talk to firewalld.