More for https://github.com/MusicPlayerDaemon/MPD/issues/42

Use the "comments" attribute instead of relying on the nullptr
terminator.

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.

Since we switched from autotools to Meson in commit
94592c14, we don't need to include
`config.h` early to properly enable large file support.  Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.

This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.

Move code from the Vorbis and FLAC decoder plugins.

To replace g_strdup().

Replaces GLib's g_ascii_strncasecmp().

Reduce heap allocator overhead.