SSL regression test suite

While working on the SSL refactoring patch, it struck me that we don't
have any regression tests for SSL support. A suite to test all the
different sslmodes etc. is essential before we can start implementing
alternatives to OpenSSL.

Now that we use TAP for testing client tools, I think we can use that to
test various SSL options too. I came up with the attached. Comments?

It currently assumes that the client's and the server's hostnames are
"postgres-client.test" and "postgres-server.test", respectively. That
makes it a bit tricky to run on a single systme. The README includes
instructions; basically you need to set up an additional loopback
device, and add entries to /etc/hosts for that.

It would make sense to separate the client and server portions of the
test so that you could run the server on one host and the client on
another. That's a TODO; I'm not sure how to do that.

- Heikki