remove the unused FlushDevice() function from the backend interface and
all interfaces.

Instead of building unix_file_device/win32_file_device directory into
libbareossd, this patch builds them as a dynamic backend and removes
special handling for them.
Also move all backend build configuration into the backends directory.

This patch removes the remaining traces of these three removed backends
from the codebase.

Previously the device-type for a device where the type was not specified
was determined when it was first used. Now we do the check for every
device right after the configuration was loaded and fail early if
needed.

This patch turns the SD's device_type into a std::string, so we can use
arbitrary backend names without registering them in a central list of
backends.

If you configure a device, the backend will be checked when the
configuration is loaded. Thus a lot of the existing test configurations
need to be adapted to either use existing storge backends or to have
BackendDirectory configured.

This patch introduces a plugin regsitry that contains all loaded
backends. Storage backends will self-register their factory either at
startup (in case the backend is statically linked into the SD) or when
the backend is loaded.

From the backend's perspective, there's no difference if it is
statically or dynamically loaded anymore.

Finally, the loading of the backends is done right after loading the
configuration, so for every device we check the presence of its backend
and fail if it is missing or cannot be loaded.

This patch removes now unused parts of the old storage backend
interface.

Instead of returning a factory that will then provide Device*, we can
return the Device* from the PluginRegistry's factory directly. This
removes the BackendInterface layer that wrapped the factory again.

Previously the droplet tests were integrated into the sd_backend tests.
With the new backend loading, we cannot configure a droplet backend if
we did not build it, thus we need to extract the droplet tests so we can
disable them completely.

This patch refactors all uses of IsFile() into individual checks that
should do what the original author intended.

This is now implemented via SeekType. The refactored code also catches
some cases when a backend does not override a required method.

Previously IsTape() was called for display purposes. This patch displays
the configured Device Type instead.

Previously, the baseclasses for generic tape support and chunked devices
were built as shared modules. As this code is never shared, we can just
link it all into the resulting backends with no drawbacks at all.

Historically, we linked against a 3rd-party package. Thus we were using
dynamic linking.
As we're shipping libdroplet ourselves and there is no other consumer
than the droplet storage backend, it is easier to link it directly into
the backend.

As we have so many kinds of plugins, we'll just call it
ImplementationFactory as it is a factory that returns implementations.