Avoid interfering with libraries also using the shared connection.

Calling dbus_shutdown() too early will break the "udisks2" neighbor
plugin.

Fixes assertion failure.

Allows other objects to use the same single instance as well.

To get udisks2 support started, this commit contains the configure.ac
option and a "neighbor" plugin which shows block devices.  Later, this
will allow mounting removable media with a new storage plugin.

From https://github.com/CM4all/libcommon/commit/a7cef404c1f7eedff1a629da48f77491218ee281

To be used by upcoming code.

Closes #284

Closes #283

release v0.20.20

Fixes:

```
error: Source option 5 is no longer supported. Use 6 or later.
error: Target option 1.5 is no longer supported. Use 1.6 or later.
```

This mostly affects the Pulse output plugin which needs to "cork" the
stream (closes #278).

Closes #277