Found with cppcoreguidelines-avoid-non-const-global-variables
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Found with readability-uppercase-literal-suffix
Signed-off-by: Rosen Penev <rosenp@gmail.com>

The former was deprecated with C++14.

According to the C++11 and C++17 standards, both files are identical.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Found with readability-uppercase-literal-suffix
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Found with modernize-redundant-void-arg
Signed-off-by: Rosen Penev <rosenp@gmail.com>

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.

Adds missing checks to several plugins.

Replaces decoder_data() and others.

Replaces decoder_initialized().

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

The old struct config_param remains only for top-level string options.

This patch allows the user to configure the mikmod decoder plugin to loop
modules. It adds a configuration parameter to the mikmod decoder called
"loop" which can be "no" (the old behaviour, default) or "yes" to allow
modules to use backward loops.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.