If libupnp is installed in a non-standard location we must rely on the
include path provided by $(pkg-config --cflags libupnp). Relative to the
path given from that command no prefix must be used to find the respective
files.

Eliminate the std::string bloat.

[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]