Fixes assertion failure.

Allows other objects to use the same single instance as well.

To get udisks2 support started, this commit contains the configure.ac
option and a "neighbor" plugin which shows block devices.  Later, this
will allow mounting removable media with a new storage plugin.

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.

We can remove those C++11 and C++14 kludges because we require GCC 4.9
now.

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

The old struct config_param remains only for top-level string options.