Shairport Sync 5.2

This update contains new features and bug fixes and is recommended for all users.

New Features and Enhancements

A new MQTT command, queue_next, has been added -- thanks to Scott Simon for the PR:
A new queue_next MQTT command enqueues a track by its Persistent ID (hexadecimal track_id), for example: queue_next 316a7653186B6917. The command is sent as a remote control command via DACP to the player.
Please note that, like all other remote control commands, this works for Classic AirPlay only.
Added a long-overdue exit handler to enable Shairport Sync to exit cleanly — that is, to attempt to clean everything up, e.g. open files, sockets, devices, threads, etc. before exit. If cleanup isn’t finished after one second, exit will occur unconditionally.
The new exit handler is now used for normal exit, abnormal exit after fatal errors and for quit requests from the D-Bus and MPRIS interfaces.

Metadata: Restored the previous way of receiving metadata, which was inadvertently removed, for better compatibility for older users of metadata. A temporary new option in the diagnostics section of the configuration file allows you to opt for the newer plist-based metadata, which is considerably richer but not yet integrated into the D-Bus or MPRIS interfaces. Thanks to Léo for the report.
Convolution & Loudness: Fixed a number of bugs in the convolution and loudness code. The bugs were due to mixing up input and output rates and channel counts. If the input and output rates and channel counts were identical, no problem. But if rates were different, the wrong finite impulse response filter would be loaded. Many thanks to Christopher Wawak for the report and follow-up.
MQTT: Fixed byte order of the track_id published over MQTT.
Fixed a very low-level mutex-locking error that interfered with debugging.
Unfixable Error Hang: Fixed a hang when an unfixable error occurred. When an unfixable error occurs without an unfixable error handler installed, the mutex protecting Shairport Sync’s alsa backend is now unlocked to permit a clean exit. Thanks to timg7 for the report.
Updated audio_sndio.c to remove some harmless compiler warnings.

The log-to-syslog command line option and the log_output_to setting in the diagnostics part of the configuration are obsolete and are now ignored.

Fixed a broken link to CONFIGURATION FLAGS.md from ADVANCED TOPICS. Thanks to Adrián Reyes for the PR.
Updated the BUILD guide to include gcrypt in the list of packages needed when building for FreeBSD / AirPlay 2.