Finally, MPD runs on Android.  For some small value of "runs".  Very
much work left, too much to describe.

Doesn't work yet, just to get the Android port started.

Fixes build failure caused by commit d9466cad.

Prepare for providing everything in a shared library, to be embedded
in another process.

Use a listener interface instead of GlobalEvents.

Build was broken by previous commit.

Fix build failure.

This is the backend for the upcoming "mount" command.

Simplifies a lot of code, because we don't need to return both the
SongEnumerator and the InputStream.

Replaces map_to_relative_path() from Mapper.cxx.

libsmbclient is not thread-safe nor reentrant.  We must protect all
function calls with a global mutex, unfortunately.