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.

Detach the Song class completely from the public API, only to be used
by SimpleDatabase and the update thread.

Don't create an intermediate Song instance when all we want is a
DetachedSong.

Don't create a temporary Song object in handle_lsinfo().  Instead,
print all tags while parsing the remote file.

Avoid GLib.

Add a new CommandResult code called "FINISH" which, unlike "CLOSE",
will attempt to flush the output buffer.  This is a one-shot attempt;
it will do one write, and not try again.

This reverts commit a577944a.

Will be replaced by new options for the old commands "search" and
"find".

Implements Mantis ticket 3843.

Check was the wrong way around, and made all valid values invisible.

This command was removed by commit 206392ad (MPD 0.16), even though it
was been proven useful for some very simple clients.  On request, I
add it to the protocol again.

Replaces GLib's g_ascii_strcasecmp().