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.

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

These are used by DSD-streaming servers.  For example, MiniDLNA uses
"audio/x-dsd".

Error message sent to client was "basic_string::_M_construct null not
valid" due to passing nullptr to the std::string constructor.

Regression caused by commit 386688b8

release v0.20.19

Closes #258

Fixes "search already in progress" errors.

See code comment.  Closes #260

Don't reactivate the PCM device immediately after Cancel() is
finished; if Cancel() gets called this may mean that new data may take
a while to produce, or no data at all will be produced because the
current song is being stopped.

Once new data is available, Play() will automatically reactivate the
PCM.

This fixes underruns when switching songs manually (closes #264).

Use the "active" flag in PrepareSockets() instead of LockHasError().

Prepare to make the "active" attribute thread-safe.