Rename struct Encoder to PreparedEncoder, and add a new (abstract)
class Encoder which represents one encoder instance.

Copy to encoder->audio_format *after* adjusting the sample format to
S24_P32.

Fixes http://bugs.musicpd.org/view.php?id=4433

The old struct config_param remains only for top-level string options.

.. and move the plugins to src/encoder/plugins/.

Replaces GLib's GError.

Add the encoder_plugin method end().  This is important for the
recorder plugin.

This fixes a buffer corruption bug; pcm_buffer is not designed to be a
persistent buffers, and will discard anything between two consecutive
calls.

These fixes were mostly generated with `codespell' [0] and manually
reviewed.

[0] http://git.profusion.mobi/cgit.cgi/lucas/codespell/

Remove trailing whitespace found by this command:
find -name '*.[ch]' | xargs grep "[[:space:]]$"

This patch prepares support for floating point samples (and probably
other formats).  It changes the meaning of the "bits" attribute from a
bit count to a symbolic value.