Exposing an installation's default value of unix_socket_directory

I just noticed that there doesn't seem to be any good way of finding
out what a postmaster's default value of unix_socket_directory is.
If you try to SHOW it you just get an empty string.  We could probably
fix things so that SHOW exposes the actual setting, but (1) there might
be security arguments against that, and (2) if your problem is that you
would like to find out the value so's you can connect to said
postmaster, SHOW isn't going to help you.

One possible response would be to add an item to what pg_config knows
about, eg "pg_config --socketdir".  This doesn't answer every possible
use-case either, but it would be helpful for some scenarios.

Thoughts?
        regards, tom lane