subtransaction cancellation in HS

Hi Simon, Hi all,

I am not sure this is 9.0 material - even if I would like it to get 
included... If HS wouldnt be new I wouldnt even consider suggesting it, but as 
its stands its a pretty small change...

It simply allows queries involving subtransaction not to get FATALed but 
canceled. This works by doing the recursive abort you erlier did in 
ProcessInterrupts in the PostgresMain where there cannot be any references 
higher up in the chain.

It would likely be sensible to check that errorcode in some PLs. I have code 
for that but I dont think its sensible to continue on those before the 
approach is agreed uppon.

I would like to get the part about a seperate error code for HS cancellations 
to get commited independently. Its kinda sensible for a client to accept 
specifically about such a cancellation and requiring them to play around with 
the message...
Currently its called ERRCODE_QUERY_CANCELED_HS but perhaps 
ERRCODE_QUERY_CANCELED_STANDBY_CONFLICT or such would be better. I dont really 
know how errorcodes gets assigned, so I picked one which is likely wrong...

Additionally there is a very small cleanup removing the errno saving from 
RecoveryConflictInterrupt - its somewhat incomplete (I think harmlessly 
though)  and more importantly the only caller in procsignal.c already does 
that...

Andres