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'"

first call fchmod() to prevent TOCTTOU, then apply permissions using
chmod()

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.

With a "+" or "-" prefix, the "metadata_to_use" setting can manipulate
the existing default.  This way, one can enable `TAG_COMMENT` without
having to list all the other tags.

This call was broken since commit
9a5bcc6d because the
`UniqueSocketDescriptor` had already been moved.

Fixes `-Werror=odr`

Changed listen_add_config_param() to use generic address parsing

the most notable bugs are 

1. osx_output_set_device_format should use the target asbd rather than AudioFormat. This is because asbd's sample rate calculation reflects the real dop target rate of the DAC, white AudioFormat's sample rate is the original DSD format rate.

2. the original code value the highest rate that's the multiple of the target rate. This cause DOP always have the wrong rate chosen. This is also not necessary for PCM playback --- MPD's goal is bit perfect, and it's meaningless to raise to two or four times the PCM sample rate.

3.  if sample_rate cannot be synchronized, the test for falling back to PCM is wrong. If the file format is in DSD format such fallback is necessary, whatever the params.dop setting is.