SSL over Unix-domain sockets

I found an old patch on my disk to enable SSL over Unix-domain sockets.

Remember, about a year ago it was discussed that there might also be
man-in-the-middle or fake-server attacks using Unix-domain sockets,
because usually anyone can start a server in /tmp. After an extensive
discussion (mainly about moving the socket out of /tmp by default;
please don't start that again), it was determined that using SSL server
verification would be the proper solution and it fact works without
problems. Except that the start-up overhead was increased significantly
(because of the initial key exchange and session key setup etc.).

Back then we didn't really have a good solution, but I figured since 8.4
rearranges the SSL connection parameters anyway, we could stick that in
there.

I imagine for example, we could invent an additional sslmode of the sort
prefer-but-not-if-local-socket, which could be the default.

The other question is whether sslverify=cn makes sense, but that may be
up to the user to find out.

Comments?