neighbor: new subsystem to detect file servers on the local network

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.

src/Idle.hxx

@@ -59,6 +59,9 @@ static constexpr unsigned IDLE_SUBSCRIPTION = 0x200;
 /** a message on the subscribed channel was received */
 static constexpr unsigned IDLE_MESSAGE = 0x400;
 
+/** a neighbor was found or lost */
+static constexpr unsigned IDLE_NEIGHBOR = 0x800;
+
 /**
  * Adds idle flag (with bitwise "or") and queues notifications to all
  * clients.