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

With "ok==false", newly created threads may quit instantly.

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

When uri_apply_base() was moved from db/upnp/Util.cpp to
util/UriUtil.cpp, the parameter order was changed, however without
swapping the parameters in the ContentDirectoryService constructor.

Necessary for "final" on gcc 4.6.

Eliminate class LibUPnP and move the code to ClientInit.cxx.  Its
initialization function can be called multiple times, but
UpnpRegisterClient() is called at most once.

Allow calling UpnpGlobalInit() multiple times.

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.