This is not a plugin.

Those are loaded with the "curl" input plugin, and this one is
"expensive", because it needs to send a new HTTP request with Range
header for each seek.

The absolute udisks:// URI is not usable with InputStream::Open(), and
thus we need to return the LocalStorage::MapUTF8() return value
instead.

Documentation will follow soon.

Avoid interfering with libraries also using the shared connection.

Calling dbus_shutdown() too early will break the "udisks2" neighbor
plugin.

Fixes assertion failure.

Allows other objects to use the same single instance as well.

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.