Remove the error message and instead erase the old setting if the
"repeatable" flag is not set.

https://github.com/MusicPlayerDaemon/MPD/issues/684

Closes https://github.com/MusicPlayerDaemon/MPD/issues/708

It appears that [[fallthrough]] is valid in C++ but not in C. And
in some Clang versions (e.g. Clang 11 on macOS), Clang is pedantic
about this and considers it an error to use [[fallthrough]] in a
.c file such as src/util/format.c.

This changes makes gcc_fallthrough a no-op under Clang in C files.

This prepares the migration away from strptime() for Windows
portability.

But the real reason I'm doing this is that strptime() on Apple is
buggy: strptime("14", "%H%M%S") (without separating colons) succeeds
even though only the hour has been parsed.  This fixes recent Travis
failures in the ParseISO8601() unit test.

For simplicity, this commit removes a workaround for an old CURL bug.

This can cause request completion in the I/O thread before this
constructor returns, leaving the object in an abstract state, causing
a crash due to pure virtual method call.  We should not start the
request until this object is fully constructed.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/665

The ALSA "null" driver opens /dev/null and returns the file handle
from snd_pcm_poll_descriptors(), but /dev/null cannot be used with
epoll, the epoll_ctl() system call returns -EPERM.  This means that
the ALSA output hangs, eventually freezing the whole MPD process.

This commit adds a workaround to the MultiSocketMonitor class which is
used by the ALSA output plugin.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/695

Closes https://github.com/MusicPlayerDaemon/MPD/issues/685

And allow "Z" suffix after date.

Most importantly, this commit translates ZZIP_ENOENT to
std::system_error(ENOENT) so IsFileNotFound() returns true and
find_stream_art() can suppress the log line.

Apparently, Icecast defaults to ISO-8859-1 for MP3:

 http://icecast.org/docs/icecast-2.4.0/config-file.html#mountsettings

This change forces Icecast to UTF-8 without having to configure it in
Icecast's configuration file.

Fixes build failure in StickerCommands.cxx.

This is the correct thing to do for ports from which a signal ultimately
"originates", such as is the case with mpd.

Commit 13208bf5 added range support to
the `move` command, but applied the wrong offset to the `to` variable.
When the source range is before the current song, and the song thus
gets decremented by the range size, then the final destination offset
must also be decremented by the range size.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/663

Commit 29f78b18 continued.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/662

Without requesting the property, "good" WebDAV servers would not send
it, and so MPD could never recognize a directory, failing the database
update.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/660

Add a runtime sanity check to avoid the assertion failure.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/660

Apparently, libmpcdec sets gain/peak variables to zero if they are not
present.  This clashes with our formula and results in bogus values
which cause noise during playback.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/640

The formula did not consider the channel count.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/639