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.

Closes #155

libupnp provides seconds, not whatever time unit is used by
std::chrono::steady_clock.

Necessary for "final" on gcc 4.6.

Each Upnp*Async() call passes a new cookie pointer, and the cookie
passed to UpnpRegisterClient() appears to be ignored.  Using this
interface is a more elegant approach than having one single "handler"
function.

There will only be very few items, and that does not justify the bloat
of std::map.

Move code to method Start() and add Error& parameters to several
methods.

Forward the Error to the caller.

Reduce bloat.

Pass "const char *" to Expat.

Remove the ContentDirectoryDescriptor right away.  Reduces bloat.

Eliminate global variables.

Delete the object when closing the database.

Delete the object when closing the database.

[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]