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

Bugs found by Coverity.

We can do CURL requests asynchronously, and we don't need a
synchronous WorkQueue thread for that.

This allows parallelizing lookups and allows immediate cancellation.

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.