SID files are generally collections of tunes, so a SID name field makes
sense as an MPD album. The SID tune information list (STIL) has name
and title fields for individual tunes, when such are known, but MPD is
currently not using the STIL.

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.

Genres are not part of the SID format, so SID files are genreless. This
"default_genre" option may be used to assign a default genre to all SID
music, for example "SID", "C64", "Chiptune", etc.

Add a `constexpr` constructor and several `constexpr` methods to
construct a DecoderPlugin at compile time, in a way which allows
adding new methods later without having to edit each plugin.

Eliminates a number of allocations, because callers don't need to copy
the strings to a newly allocated buffer only to null-terminate them.
And most callers don't need to have a null-terminated string.

Closes #202

Adds missing checks to several plugins.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

https://bugs.musicpd.org/view.php?id=4665

Speed up container_scan() again, by eliminating the need to call
scan_file() for each item.

Broken by commit 0abee77e

Replaces decoder_data() and others.

Replaces decoder_initialized().

Prepare for a Decoder API redesign based on an abstract class with
virtual methods.