Treat ENOTDIR as ENOENT when looking for client certificate file

This makes it possible to use a libpq app with home directory set
to /dev/null, for example - treating it the same as if the file
doesn't exist (which it doesn't).

Per bug #6302, reported by Diego Elio Petteno

src/interfaces/libpq/fe-secure.c

@@ -1013,7 +1013,7 @@ initialize_SSL(PGconn *conn)
 		 * might or might not accept the connection.  Any other error,
 		 * however, is grounds for complaint.
 		 */
-		if (errno != ENOENT)
+		if (errno != ENOENT && errno != ENOTDIR)
 		{
 			printfPQExpBuffer(&conn->errorMessage,
 			   libpq_gettext("could not open certificate file \"%s\": %s\n"),