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.

Reduce bloat.

Avoid appending to an obsolete string object.

Reduce overhead.

Forward the Error to the caller.

Pass "const char *" to Expat.

Reduce runtime bloat.

Reduce overhead by omitting the part of the buffer that consists only
of whitespace.

[mk: renamed source files, applied coding style, reduced bloat, using
MPD's threading library, using MPD's error reporting and logging
library and refactoring, fixed lots of bugs]