pg_stop_backup() v2 incorrectly marked as proretset

Hi all,

In my hunt looking for incorrect SRFs, I have noticed a new case of a
system function marked as proretset while it builds and returns only
one record.  And this is a popular one: pg_stop_backup(), labelled
v2.

This leads to a lot of unnecessary work, as the function creates a
tuplestore it has no need for with the usual set of checks related to
SRFs.  The logic can be be simplified as of the attached.

Thoughts?
--
Michael