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.

This invalidated the std::string instance, and thus broke the CUE
parser (commit 322b0616).

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.

This appears to be the right thing to do when we want anonymous login.

Merge duplicate code.

.. and move the plugins to src/encoder/plugins/.

This file has been unmaintained for 5 years, and nowadays, MPD tries
very hard to stay compatible with older versions.

There will only be very few items, and that does not justify the bloat
of std::map.

Fixes linker failure.

Reduce bloat.