release v0.20.21

`$srcdir/.git` doesn't exist if `$srcdir` is a worktree.

This was missing in commit 8266ab55 for #69.

The number of MPD installs on Android < 5.0 is negligible, and that
API version introduces lots of useful features for MPD.

Prevent Meson from running Android-x86 binaries.  That will fail
because the Android standard libraries are most likely not installed.

The Curl plugin requires libexpat in order to work these days, so we should
download and build it in order to get the plugin enabled on Windows.

Fixes problems with the Windows build because `-DCURL_STATICLIB` was
missing, causing error messages like:

 "undefined reference to `__imp_curl_slist_free_all'"

According to http://wiki.hydrogenaud.io/index.php?title=Cue_sheet FLAC
files should use the "WAVE" file type, but I recently encountered CUE
files declared as "FLAC" which could not be read by MPD.

Required according to RFC 3986:

> An implementation should accept uppercase letters as equivalent to
> lowercase in scheme names

Closes #330

This typo was present since the option was added in commit ed001e0c

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