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 reverts commit a577944a.

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

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 GError.

Allow the plugin to allocate the GetSong() return value.

Duplicate the music_dir variable: one encoded in UTF-8, and another
one using the configured filesystem character set.  This fixes an
ancient MPD bug.