To get udisks2 support started, this commit contains the configure.ac
option and a "neighbor" plugin which shows block devices. Later, this
will allow mounting removable media with a new storage plugin.
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.
