This way, IsFileNotFound() can detect status 404.

Allows callers to extract the NFS error code.

Make all uses of MusicChunk safe.

This adds a bit of overhead, but also adds flexibility to the API,
because arbitrary triggers may be invoked from that virtual method
implementation, not just Cond::signal().

The motivation for this is to make the handlers more dynamic, for the
upcoming buffering class utilizing ProxyInputStream.

This is not a plugin.

Documentation will follow soon.

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

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.

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 commit also puts an instance of RemoteTagScanner into the
Instance class, and hooks it into the "add" and "addid" commands.