Fix incorrect error message for IDENTIFY_SYSTEM

Commit 5a991ef869 accidentally reversed the order of the tuples
and fields parameters, making the error message incorrectly refer
to 3 tuples with 1 field when IDENTIFY_SYSTEM returns 1 tuple and
3 or 4 fields. Fix by changing the order of the parameters.  This
also adds a comment describing why we check for < 3 when postgres
since 9.4 has been sending 4 fields.

Backpatch all the way since the bug is almost a decade old.

Author: Tomonari Katsumata <t.katsumata1122@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Bug: #18224
Backpatch-through: v12

src/backend/replication/libpqwalreceiver/libpqwalreceiver.c

@@ -381,6 +381,10 @@ libpqrcv_identify_system(WalReceiverConn *conn, TimeLineID *primary_tli)
 						"the primary server: %s",
 						pchomp(PQerrorMessage(conn->streamConn)))));
 	}
+	/*
+	 * IDENTIFY_SERVER returns 3 columns in 9.3 and earlier, and 4 columns in
+	 * 9.4 and onwards.
+	 */
 	if (PQnfields(res) < 3 || PQntuples(res) != 1)
 	{
 		int			ntuples = PQntuples(res);
@@ -391,7 +395,7 @@ libpqrcv_identify_system(WalReceiverConn *conn, TimeLineID *primary_tli)
 				(errcode(ERRCODE_PROTOCOL_VIOLATION),
 				 errmsg("invalid response from primary server"),
 				 errdetail("Could not identify system: got %d rows and %d fields, expected %d rows and %d or more fields.",
-						   ntuples, nfields, 3, 1)));
+						   ntuples, nfields, 1, 3)));
 	}
 	primary_sysid = pstrdup(PQgetvalue(res, 0, 0));
 	*primary_tli = pg_strtoint32(PQgetvalue(res, 0, 1));