Related to #196

First commit for issue #69

Instead of stopping playback (due to seek time overflow), reject the
seek command.  Closes #240

Relative negative values (with "seekcur") are still allowed, and MPD
will fix the resulting position if it turns out to be negative.  But
the "seek" and "seekid" commands use an unsigned time stamp which must
not be negative.

With Grand Central Dispatch used in Main.cxx, debug builds on macOS
crash as the IsInside() assertion gets triggered in the event loop. As
a simple fix, usage of GCD is removed. Plugging and unplugging
headphones or changes of the default output device was tested without
issues. Whatever the original commit tried to fix by GCD probably does
not need fixing anymore.

From: Christian Kröner <ckroener@gmx.net>

This just copies the necessary bits and pieces from the ALSA plugin and applies them to OSXOutput based on dop config setting. It only changes the OSXOutput plugin as needed for DoP (further changes to support additionally e.g. integer mode or setting the physical device mode require rather a complete rewrite of the output plugin).

Fortunately the Core Audio API is by default bit perfect and supports DoP with minimal changes (setting the sampling rate accordingly after ensuring that the physical mode supports at least 24 bits per channel seems to be enough). This was tested on an Amanero Combo384 device hooked up to a ES9018 DAC.

USAGE (try only on DACs that support DoP):
- Add dop "yes" option to mpdconf
- Be sure to set at least 24bits per channel before playing some DSD file (using Audio-MIDI-Setup)
- Based on the dop setting, MPD will change the sample rate as required and output DoP signal to the DAC
- Hog mode is recommended to ensure that no other program will try to mix some output with the DoP stream (resulting in bad noise)
- Alternatively set the default output device to another device (e.g. the built-in output) to avoid having other audio interfere with DSD playback

support for chaining ogg opus streams to enable changing stream' metadata on the fly.
currently support is opt-in (enabled by additional option) because lots of clients can't handle this properly yet.

release v0.20.18

Now that all build failures have been fixed, we can enable the
feature.

Fixes two build failures with libnfs on Windows.

configure.ac sets this, but this wasn't used for compiling third-party
libraries.  This setting however is important for libnfs, which adds
fallback definitions for POLLIN and POLLOUT with bogus values.

It indeed is a macro on Windows.

Instead of setting state=IGNORE_TRACK, ignore only the following
"INDEX" lines.

Correction for commit 8461d71b.  Closes #227

Let FFmpeg do the MP3 decoding.  See commit
a4de9650

Closes #232

libmad has been unmaintained for a long time, and it fails to build on
Windows.  I could go and fix libmad's broken configure script, but I
prefer to just assign MP3 decoding to FFmpeg for now.

Closes #228

Enable the NFS storage plugin on Android.

Closes #226

read_stream_art uses PRIu64 unconditionally with the Format
method of a Respone instance to output a size_t typed value.
If size_t is 32bit the output is garbeled. This patch uses
offset_type and PRIoffset to make sure the format string
and the type of the output value always match.

This addresses two problems:

1. the libFLAC write callback had to send an error status to its
caller when SubmitData() returned a command; this disrupted libFLAC
and the resulting command could not be used for anything;

2. the libFLAC function FLAC__stream_decoder_seek_absolute() also
calls the write callback, but its result cannot be used, because
seeking is still in progress, so we lose all data from one FLAC frame.
By moving the SubmitData() call until after CommandFinished(), we
avoid losing this data.  This fixes another part of #113

See code comment.