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

This header had been available for a long time on Linux, but was
removed in glibc 2.30.  This commit moves the `#include` line inside
the `#ifdef __sun` block and adds a fake declaration of `I_FLUSH` for
the Linux build.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/630

The new Response instance in the `catch` block didn't have the
`command` attribute set, so the error response didn't indicate which
command had failed, which however is required in the MPD protocol.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/628

Clarify that those can't throw, preparing for the next commit.

Previously, a bogus value (whatever happened to be still in
`start_time`) was used.

See code comment.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/629

This was missing in commit 0f488dce

Field 2 is called <released>, formerly used as <copyright>[1][2]. It is
formatted <year><space><company or author or group>, where <year> may be
<YYYY>, <YYY?>, <YY??> or <YYYY-YY>, for example "1987", "199?", "19??"
or "1985-87". The <company or author or group> may be for example Rob
Hubbard. A full field may be for example "1987 Rob Hubbard".

This change splits the <released> field at the first <space>, to retain
the <year> part.

The 51823 SID files in High Voltage SID Collection (HVSC) version 71
have the following distribution of dates:

    333 19??         11 1990-92       6 1995-99       2 2006-08
    827 198?         88 1990-93    2140 1996        530 2007
     32 1982         69 1990-94       9 1996-97      15 2007-08
      1 1982-83      49 1990-95       2 1996-98       2 2007-09
    255 1983       3467 1991          5 1996-99       1 2007-10
    677 1984         75 1991-92    1840 1997        430 2008
    775 1985         65 1991-93       4 1997-98      23 2008-09
      3 1985-86      10 1991-94    1276 1998          1 2008-12
     10 1985-87      35 1991-97       4 1998-99     631 2009
    943 1986       3320 1992        865 1999          1 2009-10
     12 1986-87      26 1992-93      24 200?        645 2010
      5 1986-89      59 1992-94     590 2000          1 2010-12
   2083 1987          1 1992-96       4 2000-01     538 2011
     31 1987-88    2996 1993        727 2001          1 2011-12
     44 1987-89      42 1993-94     875 2002        651 2012
   2510 1988         12 1993-95       2 2002-04     811 2013
    129 1988-89       2 1993-97     844 2003        790 2014
     91 1988-90    2737 1994          3 2003-05     740 2015
     58 1988-91      16 1994-95     842 2004        792 2016
   3466 1989         20 1994-96       2 2004-05     775 2017
     95 1989-90      17 1994-97     707 2005        638 2018
    150 1989-91    2271 1995          1 2005-06     284 2019
   1077 199?          2 1995-96       2 2005-07
   2834 1990          4 1995-97     785 2006
    119 1990-91       2 1995-98       6 2006-07

References:

[1] https://www.hvsc.c64.org/download/C64Music/DOCUMENTS/SID_file_format.txt
[2] https://hvsc.c64.org/info

High Voltage SID Collection (HVSC) metadata fields are encoded in
windows-1252, as described in DOCUMENTS/SID_file_format.txt:

https://www.hvsc.c64.org/download/C64Music/DOCUMENTS/SID_file_format.txt

If utf-8 transcoding fails, or the ICU library is unavailable, fall
back to plain ASCII and replace other characters with '?'.

The song length was previously undetermined.

Without this, the compiler may optimize accesses away.

Version 10.2+0.93+1 was released five years ago in 2014 and is the
first version to feature cdio_cddap_free_messages().  There is no way
to check the libcdio-paranoia version at compile time, so let's just
remove support for older versions instead of attempting to fix the
cdio_cddap_free_messages() check at build time.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/613

This reverts commit f7ed7446.  It was
a bad idea, because MAD_F_MIN and MAD_F_MAX do not represent the
clamping limits, but the theoretical minimum and maximum values of the
mad_fixed_t data type.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/617