This way, IsFileNotFound() can detect status 404.

Supports .mpdignore on NFS/SMB and others (closes #290).

Allows callers to extract the NFS error code.

See commit be137a19

Since 7d353bbe, _GNU_SOURCE is always
defined, which implies __USE_MINGW_ANSI_STDIO and thus switches to
the mingw implementations of the printf() family.  That's
standards-compliant, unlike Microsoft's CRT implementations.

This code was added in 21851c06 but
looks completely broken:

- the status code is "206 OK" but "206" would be "Partial Content"

- the "Content-Length" header has a bogus value

- the "Content-RangeX" parameter has different bogus values (why
  "Content-RangeX" anyway and not "Content-Range"?)

Apart from that, there are strange undocumented non-standard headers
which are probably there to work around bugs/expectations in one
broken proprietary client product.  But these days, MPD doesn't bend
over to support broken clients.  So let's kill this code.

Closes #304

Closes #284

Closes #283

Fixes:

```
error: Source option 5 is no longer supported. Use 6 or later.
error: Target option 1.5 is no longer supported. Use 1.6 or later.
```

This mostly affects the Pulse output plugin which needs to "cork" the
stream (closes #278).

Closes #277

These are used by DSD-streaming servers.  For example, MiniDLNA uses
"audio/x-dsd".

Error message sent to client was "basic_string::_M_construct null not
valid" due to passing nullptr to the std::string constructor.

Regression caused by commit 386688b8

Closes #258

Fixes "search already in progress" errors.

av_register_all() was deprecated in
FFmpeg/FFmpeg@0694d8702421e7aff1340038559c438b61bb30dd

When switching to another song manually, the player checks if the
decoder is already decoding that song; if so, it will attempt to reuse
it by seeking it to the new position.  That however fails if the
decoder is not seekable (e.g. a radio stream) which leaves the user
unable to switch to that song with the bogus error message "Not
seekable".

There was a discrepancy between what was written to the buffer and the
size returned by pcm_dsd_to_dop(): the "for" loop uses num_frames/2,
rounding down, while the return value is num_samples which is
num_frames*channels, without rounding.  This could cause undefined
data at the end of the destination buffer if the source buffer size
was not aligned to multiples of 8 bytes (4 DSD bytes per channel).

The latter however can occur in the 0.21 branch after commit
a06bf388

Closes #233