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

Since the encoder plugin uses a libvorbis function (and not only
libvorbisenc functions), we need to link with libvorbis explicitly.

Passing a void pointer is unsafe.

This commit finally allows the database update to scan remote files,
e.g. using the smbclient storage plugin.  However, it is not yet
possible to configure that, therefore the feature is not accessible
yet.

Prepare for the plugin interface.

Keep only a reference.

Fixes bogus assertion failure.

Prepare to make this a new plugin API, for example to use a SMB share
for the music_directory.

Fixes assertion failure when update gets launched during MPD startup.

Replaces global variable "main_loop".

Decouples db/update/Remove.cpp from global variables.