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

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.

Variable-length arrays are a C-only feature.

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.

Prepare for templated functions.

Just like std::condition_variable, which however has no way to specify
the std::mutex directly.

If the DecoderThread gets woken up by a STOP command, this should be
detected as early as possible.

Let the constructor and destructor do this.  This means that all users
have to be converted to allocate PcmConvert dynamically.

This plugin is interesting only for a tiny fraction of MPD users, so
let's not spam everybody else's log with it.