Closes #193

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.

libupnp 1.6.24 added a few badly designed macros which break the MPD
build:

 https://sourceforge.net/p/pupnp/code/ci/8177a4195a4d4a5d6cb095eaf4ca5b65ac446381/

To work around this, we disable our emulation functions (from
714011c8) on this libupnp version.

Closes #163

Closes #155

Bugs found by Coverity.

Got lost in commit e5ee3579

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.

memcmp() can overflow the buffer.

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.