Fix build breakage from commit ea639269

Closes https://github.com/MusicPlayerDaemon/MPD/issues/589

This makes the configuration error more visible, possibly addressing
one part of https://github.com/MusicPlayerDaemon/MPD/issues/589

Workaround for a regression caused by commit
a06bf388, revealing a problem with
discarding odd numer of frames in the DSD_U32 and DoP converters,
causing distortions with DSD_U32 and DoP on 32 bit CPUs.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/469

This can happen if the DoP converter doesn't get enough source samples
for one destination quad.  This isn't a critical bug, because the OSS
plugin doesn't support DoP yet, but it's good to be prepared.

Instead of passing tag and group, pass an array of tags.  To support a
nested return value, return a nested std::map of std::maps.  Each key
specifies the tag value, and each value may be another nesting level.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/408

Since we now don't duplicate all items, we can easily remove the 64kB
limit from OpusReader::ReadString() and instead silently ignore and
skip all strings which are longer than 4 kB.

This fixes a tag duplication bug with Opus file containing a very long
`METADATA_BLOCK_PICTURE` tag, which occurred because the Opus plugin
returned false after parsing all tags, and then the MPD core fell back
to FFmpeg which scanned the tags again.

Return `404 not found` for some common well-known paths, as clients requesting them usually do that automatically and don't expect endless audio stram.

Closes #572

memcmp use may result in out of bounds access

This reverts commit ff3e2c05.  The
check was necessary, after all, because this is what checked whether
the decoder had finished the current or the next song.

> The "queued" flag can only possibly be set if the decoder is still
> decoding the current song or if the decoder is stopped.

That was wrong because ProcessCommand() sets `queued=true` and also
starts the decoder (if it was idle).

> This is also what the following assert() checks.

That was also wrong, because the assert() has two conditions.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/566

If the decoder finishes decoding the current song between the two
IsIdle() checks, MPD stops playback instead of starting the decoder
for the next song.

This is usually not visible problem, because the main thread restarts
it via playlist::ResumePlayback(), but that way it, ignores "single"
mode.

As a workaround, this commit adds another "queued" check which
re-enters the player loop and checks again whether to start the
decoder.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/556

The "queued" flag can only possibly be set if the decoder is still
decoding the current song or if the decoder is stopped.  This is also
what the following assert() checks.  This check was not necessary.

This enables the input buffer for remote files and caches file
contents in MPD.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/376

Commit b3a45833 added a LocateUri()
call to several playlist commands, which applied InputPlugin URI
scheme verification to playlist URIs.  This broke the SoundCloud
playlist plugin which uses "soundcloud://" URIs for which no input
plugin exists.

This commit allows the caller to specify the kind of plugin which
shall be used to verify the URI.  Right now, only "input" is
implemented; "storage" uses the "input" verification for now; and
"playlist" has no verification at all (for now).

Closes https://github.com/MusicPlayerDaemon/MPD/issues/528

There is already one call in ReadServers(), which is the correct place
to do it.

Split several printf() calls to make it easier to deal with all those
#ifdefs.

Expect OnSocketReady() to cancel events.  If it returns false, the
SocketMonitor may be destructed already.  This fixes a use-after-free
bug in the "httpd" output plugin.

Keep the SocketMonitor registered.  This wrong return value was added
6 years ago in commit 72cf8dd8, andd
apparently, nobody ever noticed.

This missing piece probably never really hurt, because
HttpdClient::OnSocketClosed() would be called right after a socket
error, but it's better to be explicit about closing on error.

Fixes:

 src/net/IPv4Address.hxx: In member function 'constexpr IPv4Address::operator SocketAddress() const':
 src/net/IPv4Address.hxx:171:24: error: a reinterpret_cast is not a constant expression
   171 |   return SocketAddress((const struct sockaddr *)&address,
       |                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

 src/net/IPv6Address.hxx: In member function 'constexpr IPv6Address::operator SocketAddress() const':
 src/net/IPv6Address.hxx:138:24: error: a reinterpret_cast is not a constant expression
   138 |   return SocketAddress((const struct sockaddr *)&address,
       |                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Closes https://github.com/MusicPlayerDaemon/MPD/issues/522

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.