Redefine default result from PQhost()?

Currently, libpq's PQhost() function will return NULL if the connection is
using the default Unix-socket connection address.  This seems like a
pretty dubious definition to me.  psql works around it in several places
with
           host = PQhost(pset.db);           if (host == NULL)               host = DEFAULT_PGSOCKET_DIR;

That seems rather duplicative, and it also means that both libpq and psql
have the default socket directory hard-wired into them, which does not
seem like a good thing.  It's conceivable that psql could get used with
a libpq built with a different default, in which case these places would
be outright lying about which socket is being used.

I think we should do this:
char *PQhost(const PGconn *conn){    if (!conn)        return NULL;    if (conn->pghost != NULL && conn->pghost[0] !=
'\0')       return conn->pghost;    else    {#ifdef HAVE_UNIX_SOCKETS
 
-        return conn->pgunixsocket;
+        if (conn->pgunixsocket != NULL && conn->pgunixsocket[0] != '\0')
+            return conn->pgunixsocket;
+        else
+            return DEFAULT_PGSOCKET_DIR;#else        return DefaultHost;#endif    }}

As a definitional change, this would be for HEAD only.

Comments?
        regards, tom lane