Prepare for the plugin interface.

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

Move from db/DatabaseGlue.cxx, eliminating global variable.

There was quite a lot of duplicate code for loading DetachedSong
objects, with different semantics for "securely" loading local files.

Move global variables into the new classes.  That may allow multiple
update threads for multiple databases one day.

Decouple command/OtherCommands.cxx from the "simple" database plugin.

Fixes non-Linux build without neighbor plugins.

Another big chunk of code for multi-player support.

We're C++, and sparse understands only plain C.

Was broken on out-of-tree builds.

Eliminate class LibUPnP and move the code to ClientInit.cxx.  Its
initialization function can be called multiple times, but
UpnpRegisterClient() is called at most once.

Allow calling UpnpGlobalInit() multiple times.

Each Upnp*Async() call passes a new cookie pointer, and the cookie
passed to UpnpRegisterClient() appears to be ignored.  Using this
interface is a more elegant approach than having one single "handler"
function.

Simplifies test program rules.

This commit adds the NeighborPlugin API which can be used to detect
nearby file servers that can be used by input plugins.  This list of
servers is exported using the new "listneighbors" command.  The idle
even "neighbor" notifies interested clients when a new neighbor is
found or an existing one is lost.

There's a lot missing currently: protocol&user documentation, and a
way to "mount" remote servers into the music database.  Obviously,
some code from the UPnP database plugin can be moved to a neighbor
plugin.