Fix memory leak in LogStandbySnapshot().

The array allocated by GetRunningTransactionLocks() needs to be pfree'd
when we're done with it.  Otherwise we leak some memory during each
checkpoint, if wal_level = hot_standby.  This manifests as memory bloat
in the checkpointer process, or in bgwriter in versions before we made
the checkpointer separate.

Reported and fixed by Naoya Anzai.  Back-patch to 9.0 where the issue
was introduced.

In passing, improve comments for GetRunningTransactionLocks(), and add
an Assert that we didn't overrun the palloc'd array.
Add ARM64 (aarch64) support to s_lock.h.

Use the same gcc atomic functions as we do on newer ARM chips.
(Basically this is a copy and paste of the __arm__ code block,
but omitting the SWPB option since that definitely won't work.)

Back-patch to 9.2.  The patch would work further back, but we'd also
need to update config.guess/config.sub in older branches to make them
build out-of-the-box, and there hasn't been demand for it.

Mark Salter
Provide better message when CREATE EXTENSION can't find a target schema.

The new message (and SQLSTATE) matches the corresponding error cases in
namespace.c.

This was thought to be a "can't happen" case when extension.c was written,
so we didn't think hard about how to report it.  But it definitely can
happen in 9.2 and later, since we no longer require search_path to contain
any valid schema names.  It's probably also possible in 9.1 if search_path
came from a noninteractive source.  So, back-patch to all releases
containing this code.

Per report from Sean Chittenden, though this isn't exactly his patch.
Put analyze_keyword back in explain_option_name production.

In commit 2c92edad48796119c83d7dbe6c33425d1924626d, I broke "EXPLAIN
(ANALYZE)" syntax, because I mistakenly thought that ANALYZE/ANALYSE were
only partially reserved and thus would be included in NonReservedWord;
but actually they're fully reserved so they still need to be called out
here.

A nicer solution would be to demote these words to type_func_name_keyword
status (they can't be less than that because of "VACUUM [ANALYZE] ColId").
While that works fine so far as the core grammar is concerned, it breaks
ECPG's grammar for reasons I don't have time to isolate at the moment.
So do this for the time being.

Per report from Kevin Grittner.  Back-patch to 9.0, like the previous
commit.
Update SQL features list
Prevent pushing down WHERE clauses into unsafe UNION/INTERSECT nests.

The planner is aware that it mustn't push down upper-level quals into
subqueries if the quals reference subquery output columns that contain
set-returning functions or volatile functions, or are non-DISTINCT outputs
of a DISTINCT ON subquery.  However, it missed making this check when
there were one or more levels of UNION or INTERSECT above the dangerous
expression.  This could lead to "set-valued function called in context that
cannot accept a set" errors, as seen in bug #8213 from Eric Soroos, or to
silently wrong answers in the other cases.

To fix, refactor the checks so that we make the column-is-unsafe checks
during subquery_is_pushdown_safe(), which already has to recursively
inspect all arms of a set-operation tree.  This makes
qual_is_pushdown_safe() considerably simpler, at the cost that we will
spend some cycles checking output columns that possibly aren't referenced
in any upper qual.  But the cases where this code gets executed at all
are already nontrivial queries, so it's unlikely anybody will notice any
slowdown of planning.

This has been broken since commit 05f916e6add9726bf4ee046e4060c1b03c9961f2,
which makes the bug over ten years old.  A bit surprising nobody noticed it
before now.
pg_upgrade:  document that --link should be used with --check
Backpatch to 9.2.
Ensure that XLOG_HEAP2_VISIBLE always targets an initialized page.

Andres Freund
Fix typo in comment.
Merge branch 'master' into filter
Atri's patch, pg_proc.h adjusted per previous, whitespace errors elided.
Merge branch 'filter' into within_group
Merged in conflicts