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

Replaces g_ascii_strup() and allows building the Vorbis encoder
without GLib.

Enables using range-based "for".

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

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

The MPD core will add this as a prefeix.

Replaces GLib's GError.

Improves quality by not squeezing 32 bit samples down to 16 bit, and
then back to 32 bit floating point.  Reduces CPU usage by skipping a
conversion step.

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

Don't reset the ogg_stream_state object, because this discards the
end-of-stream packet that was just added.

In the "vorbis" plugin, this is a copy of the old flush() method,
while flush() gets a lot of code remove, it just sets the "flush" flag
and nothing else.  It doesn't start a new stream now, which should fix
a few problems in some players.

Without the ogg_stream_reset() call, the "e_o_s" flag never gets
reset, and libogg writes EOS packets over and over.

Without the ogg_stream_reset() call, the "e_o_s" flag never gets
reset, and libogg writes EOS packets over and over.

Replaced all occurrences of g_error() with MPD_ERROR() located in a new header
file 'mpd_error.h'. This macro uses g_critical() to print the error message
and then exits gracefully in contrast to g_error() which would internally call
abort() to produce a core dump.

The macro name is distinctive and allows to find all places with dubious error
handling. The long-term goal is to get rid of MPD_ERROR() altogether. To
facilitate the eventual removal of this macro it was added in a new header
file rather than to an existing header file.

This fixes #2995 and #3007.