Fix issues with checks for unsupported transaction states in Hot Standby.

The GUC check hooks for transaction_read_only and transaction_isolation
tried to check RecoveryInProgress(), so as to disallow setting read/write
mode or serializable isolation level (respectively) in hot standby
sessions.  However, GUC check hooks can be called in many situations where
we're not connected to shared memory at all, resulting in a crash in
RecoveryInProgress().  Among other cases, this results in EXEC_BACKEND
builds crashing during child process start if default_transaction_isolation
is serializable, as reported by Heikki Linnakangas.  Protect those calls
by silently allowing any setting when not inside a transaction; which is
okay anyway since these GUCs are always reset at start of transaction.

Also, add a check to GetSerializableTransactionSnapshot() to complain
if we are in hot standby.  We need that check despite the one in
check_XactIsoLevel() because default_transaction_isolation could be
serializable.  We don't want to complain any sooner than this in such
cases, since that would prevent running transactions at all in such a
state; but a transaction can be run, if SET TRANSACTION ISOLATION is done
before setting a snapshot.  Per report some months ago from Robert Haas.

Back-patch to 9.1, since these problems were introduced by the SSI patch.

Kevin Grittner and Tom Lane, with ideas from Heikki Linnakangas

src/backend/storage/lmgr/predicate.c

@@ -1558,6 +1558,19 @@ RegisterSerializableTransaction(Snapshot snapshot)
 {
 	Assert(IsolationIsSerializable());
 
+	/*
+	 * Can't use serializable mode while recovery is still active, as it is,
+	 * for example, on a hot standby.  We could get here despite the check
+	 * in check_XactIsoLevel() if default_transaction_isolation is set to
+	 * serializable, so phrase the hint accordingly.
+	 */
+	if (RecoveryInProgress())
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("cannot use serializable mode in a hot standby"),
+				 errdetail("\"default_transaction_isolation\" is set to \"serializable\"."),
+				 errhint("You can use \"SET default_transaction_isolation = 'repeatable read'\" to change the default.")));
+
 	/*
 	 * A special optimization is available for SERIALIZABLE READ ONLY
 	 * DEFERRABLE transactions -- we can wait for a suitable snapshot and