Re: default_isolation_level='serializable' crashes on Windows

I wrote:
> I poked around this area a bit.  I notice that
> check_transaction_read_only has got the same fundamental error: it
> thinks it can safely consult RecoveryInProgress(), which in general
> it cannot.

After rereading the whole thread I saw that Heikki had already pointed
this out, and come to the same conclusion about how to fix it:

> On reflection I think maybe the best solution is for
> check_transaction_read_only to test IsTransactionState(), and just
> allow the change always if outside a transaction.

Attached is a version of the patch that does it like that.  I've checked
that this fixes the problem (as well as Robert's earlier report) in an
EXEC_BACKEND build, which is as close as I can get to the Windows
environment.

I tweaked Kevin's error message to keep the same capitalization as the
existing text for the message in check_XactIsoLevel --- if we change
that it will cause work for the translators, and I don't think it's
enough of an improvement to justify that.

I also back-propagated the use of ERRCODE_FEATURE_NOT_SUPPORTED into
the GUC check hooks.  On reflection this seems more appropriate than
ERRCODE_INVALID_PARAMETER_VALUE.

Lastly, I simplified the added code in InitPostgres down to just a
bare assignment to XactIsoLevel.  It doesn't seem worthwhile to add
all the cycles involved in SetConfigOption(), when we have no desire
to change the GUC permanently.  (I think Kevin's code was wrong anyway
in that it was using PGC_S_OVERRIDE, which would impact the reset
state for the GUC.)

I think this is ready to go.  Kevin, do you want to apply it?  You
had mentioned wanting some practice with back-patches.

			regards, tom lane