This adds a bit of overhead, but also adds flexibility to the API,
because arbitrary triggers may be invoked from that virtual method
implementation, not just Cond::signal().

The motivation for this is to make the handlers more dynamic, for the
upcoming buffering class utilizing ProxyInputStream.

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.