release v0.21.7

Let the caller decide what to do with the original exception.

Preserve the original exception.

This plugin is interesting only for a tiny fraction of MPD users, so
let's not spam everybody else's log with it.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/234

Fixes #184.

Semaphores are kernel-managed objects, calling delete_sem() twice is not more
dangerous than calling close() twice on an fd though, it would just return
an error.

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.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/516