Re: fix crash with Python 3.11

I wrote:
> * I'm not satisfied with using static storage for
> SaveTransactionCharacteristics/RestoreTransactionCharacteristics.

Looking closer at this, I was not too amused to discover that of the three
existing SaveTransactionCharacteristics calls, two already conflict with
each other: _SPI_commit calls SaveTransactionCharacteristics and then
calls CommitTransactionCommand, which again calls
SaveTransactionCharacteristics, overwriting the static storage.
I *think* there's no live bug there, because the state probably wouldn't
have changed in between; but considering we run arbitrary user-defined
code between those two points I sure wouldn't bet on it.

0001 attached is the same code patch as before, but now with spi.sgml
updates; 0002 changes the API for Save/RestoreTransactionCharacteristics.
If anyone's really worried about backpatching 0002, we could perhaps
get away with doing that only in HEAD.

I found in 0002 that I had to make CommitTransactionCommand call
SaveTransactionCharacteristics unconditionally, else I got warnings
about possibly-uninitialized local variables. It's cheap enough
that I'm not too fussed about that. I don't get any warnings from
the similar code in spi.c, probably because those functions can't
be inlined there.

regards, tom lane