This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Reduce bloat.  We never used the set lookup.

Fixes problems with some UPnP servers.

Reduce overhead.

Reduce overhead.

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]