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

Closes https://github.com/MusicPlayerDaemon/MPD/issues/568

According to http://www.sphinx-doc.org/en/master/usage/restructuredtext/basics.html#sections

Closes https://github.com/MusicPlayerDaemon/MPD/issues/340

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.

ALSA is just one out of many output plugins, and detailed plugin
documentation should only live in the user manual, without having
duplicates in the (brief) manpage.

Also move "mixer_type" to the "optional audio output parameters"
section; it is a generic option, not specific to ALSA.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/552

Using this API function requires SDK level 23.

Fixes nullptr dereference when an exception gets thrown because there
is no ".eh_frame" section for unwinding.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/543

Closes https://github.com/MusicPlayerDaemon/MPD/issues/526

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.

Fixes Travis failure with Meson 0.50:

 ERROR: Could not detect Ninja v1.5 or newer