Prepare for using other crypto libraries, e.g. FFmpeg's libavutil.

Time to move on, two years after 2017.

Commit 1eae9339 added support for
multiple "groups" in the "list" command, and this change allows
clients to detect that this behavior, which had been documented for
several years, is now implemented properly.

A first use case for our libchromaprint integration added by commit
30e22b75

The custom_command was run in src/haiku/ and created a file with only resources inside.

Since xres edits the file in-place and meson doesn't like it, we have to run a shell script for now.
Maybe later I'll add proper support in meson.

Meson 0.49.0 adds native support for `libgcrypt-config` which is
necessary for detecting libgcrypt dependencies, as the latest
version 1.8.4 of libgcrypt does not provide a .pc file.

There is a minor new feature (commit 713c1f2b) and clients might be
interested in detecting it by the protocol version.

Time to create a new unstable branch; stable development will now
continue in the branch v0.21.x

libwrap is an obscure artefact from a past long ago, when source IP
address meant something.

And its API is "interesting"; it requires the application to expose
two global variables `allow_severity` and `deny_severity`.  This led
to bug #437.  I don't want to declare those variables; instead, I'd
like to remove libwrap support.

Closes #437

This version should have been incremented in 0.21.2 because this
version added new search operators.  In 0.21.4, we'll have more
protocol bug fixes which may be important for clients to know.

This feature requires `libpcre`.

Some Boost 1.67 packages apparently have a workaround for the bug, so
let them build MPD.