See https://github.com/MusicPlayerDaemon/MPD/issues/545

This can be used for other purposes as well.

Let the caller do this.  Our GetSuffix() function was broken anyway.

Don't call Seek() if the stream is already at the beginning.  This
avoids unnecessary exceptions if seeking is not implemented by an
Inputstream implementation.

PluginUnconfigured exceptions are logged with level "info" instead of
"error".  This suppresses some rather boring messages in the default
log level.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/565

Another step towards supporting multiple readers.

There is just one thread.

The thread will always attempt to read more data since commit
2cf6b776, so we don't need to tell it
to continue.

The "seek_error" attribute will be eliminated soon.

More preparations to support multiple readers.

If a read error occurs, it is very unlikely that the InputStream will
ever recover.  Removing the code removes some code complexity which
just isn't worth it.  And it allows supporting multiple readers for
one buffer.

After seeking, a new read should be done.

Prepare to reuse it in another class.