Fix the build with new workspace name.

Mention that List() has no proper testing coverage.

Remove the dependency on accept().

As accept() is going to be removed from CloudABI's C library, allow the
switchboard to simply accept a socket endpoint for the initial handle.

Don't declare CreateSocket() on CloudABI.

There is no code in Flower that is supposed to build on CloudABI that
makes use of it. This allows us to throw away socket() from cloudlibc.

Make flower_cat's -l flag work.

Implement pruning of dead targets.

Now that ARPC provides a GetState() function, extend listeners to have
an IsDead() function. Extend Directory to have a PruneDeadTargets()
function that is called prior to iteration.

While there, remove locking of channels as done by the listeners. GRPC
channels are thread-safe, so we should do the same thing for ARPC.

Install the .ad.h files as well.

This makes it easier to make use of Flower in applications.

Remove support for running flower_ingress_accept on CloudABI.

Now that we're removing socket address support from CloudABI, it makes
very little sense to run this utility through CloudABI. There is no way
to fetch the labels this utility wants to add to incoming connections.

Simply use a JSON library to generate the JSON-esque strings.

Clean up flower_ingress_accept to be buildable on CloudABI.