Fixup after e4dad42c

This per-object Mutex replaces the global `smbclient_mutex`.

This is no longer necessary with the new API.

This is no longer necessary with the new API.

Preparing to replace `smbclient_mutex`, for finer-grained locking.

We no longer need to call smbc_init() because we don't need the compat
layer anymore.

As a side effect, the input plugin closes the SMB/CIFS connection
after closing the file.

This solves one part of
https://github.com/MusicPlayerDaemon/MPD/issues/916

release v0.21.25

This fixes compatibility with Plex DLNA.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/851

Stop bothering people about the Tidal/Qobuz plugins.

Make LogError()/FormatError() wrappers for those.  Now we can log
exceptions with a lower level.

Prepare for templated functions.

Print all nested exceptions on a single line to avoid confusion.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/864

Closes https://github.com/MusicPlayerDaemon/MPD/issues/841

Mounting one storage URI twice on different mount points can lead to
conflicts with the database cache file, and it doesn't make a lot of
sense.

But most importantly, our udisks storage plugin will unmount the disk
from the kernel VFS, and if two exist, they will compete with each
others.  We could (and should) fix this in the udisks storage plugin,
but for now, this workaround is good enough (and useful).

When mounting something over a directory that is already a mount
point, CompositeStorage::Mount() silently overwrites the previously
mounted storage, disposing it.  After that, SimpleDatabase::Mount()
will fail and handle_mount() will roll back the
CompositeStorage::Mount() command, effectively unmounting what was
there before (and also leaking memory).

Closes https://github.com/MusicPlayerDaemon/MPD/issues/918