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]