I noticed that in pgstatfuncs.c, the AF_UNIX macro is being used
unprotected by HAVE_UNIX_SOCKETS, apparently since 2008. So I think the
redirection through IS_AF_UNIX() is no longer necessary. (More
generally, all supported platforms are now HAVE_UNIX_SOCKETS, but even
if there were a new platform in the future, it seems plausible that it
would define the AF_UNIX symbol even without kernel support.) So maybe
we should remove the IS_AF_UNIX() macro and make the code a bit more
consistent?