PortalRun must guard against the possibility that the portal it's

PortalRun must guard against the possibility that the portal it's
running contains VACUUM or a similar command that will internally start
and commit transactions.  In such a case, the original caller values of
CurrentMemoryContext and CurrentResourceOwner will point to objects that
will be destroyed by the internal commit.  We must restore these pointers
to point to the newly-manufactured transaction context and resource owner,
rather than possibly pointing to deleted memory.
Also tweak xact.c so that AbortTransaction and AbortSubTransaction
forcibly restore a sane value for CurrentResourceOwner, much as they
have always done for CurrentMemoryContext.  I'm not certain this is
necessary but I'm feeling paranoid today.
Responds to Sean Chittenden's bug report of 4-Oct.