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.

Prepare the transition to std::mutex.

Just like std::condition_variable, which however has no way to specify
the std::mutex directly.

This enables the input buffer for remote files and caches file
contents in MPD.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/376

Let the caller decide what to do with the original exception.

Preserve the original exception.

using the device "default" brings this plugin into line with the AlsaOutputPlugin; and a sample rate of 48kHz is more widely used as a native default for modern hardware than 44.1kHz

Also fixes an inconsistency between the docs and code.

input/plugins/AlsaInputPlugin: introduce mpd.conf config block to allow user to override the builtin defaults

input/plugins/AlsaInputPlugin: extend the alsa uri parsing to permit specification of the desired pcm audio format in the uri