Repair bugs in GiST page splitting code for multi-column indexes.

When considering a non-last column in a multi-column GiST index,
gistsplit.c tries to improve on the split chosen by the opclass-specific
pickSplit function by considering penalties for the next column.  However,
there were two bugs in this code: it failed to recompute the union keys for
the leftmost index columns, even though these might well change after
reassigning tuples; and it included the old union keys in the recomputation
for the columns it did recompute, so that those keys couldn't get smaller
even if they should.  The first problem could result in an invalid index
in which searches wouldn't find index entries that are in fact present;
the second would make the index less efficient to search.

Both of these errors were caused by misuse of gistMakeUnionItVec, whose
API was designed in a way that just begged such errors to be made.  There
is no situation in which it's safe or useful to compute the union keys for
a subset of the index columns, and there is no caller that wants any
previous union keys to be included in the computation; so the undocumented
choice to treat the union keys as in/out rather than pure output parameters
is a waste of code as well as being dangerous.

Hence, rather than just making a minimal patch, I've changed the API of
gistMakeUnionItVec to remove the "startkey" parameter (it now always
processes all index columns) and treat the attr/isnull arrays as purely
output parameters.

In passing, also get rid of a couple of unnecessary and dangerous uses
of static variables in gistutil.c.  It's remarkable that the one in
gistMakeUnionKey hasn't given us portability troubles before now, because
in addition to posing a re-entrancy hazard, it was unsafely assuming that
a static char[] array would have at least Datum alignment.

Per investigation of a trouble report from Tomas Vondra.  (There are also
some bugs in contrib/btree_gist to be fixed, but that seems like material
for a separate patch.)  Back-patch to all supported branches.
Fix erroneous range-union logic for varlena types in contrib/btree_gist.

gbt_var_bin_union() failed to do the right thing when the existing range
needed to be widened at both ends rather than just one end.  This could
result in an invalid index in which keys that are present would not be
found by searches, because the searches would not think they need to
descend to the relevant leaf pages.  This error affected all the varlena
datatypes supported by btree_gist (text, bytea, bit, numeric).

Per investigation of a trouble report from Tomas Vondra.  (There is also
an issue in gbt_var_penalty(), but that should only result in inefficiency
not wrong answers.  I'm committing this separately so that we have a git
state in which it can be tested that bad penalty results don't produce
invalid indexes.)  Back-patch to all supported branches.
Make contrib/btree_gist's GiST penalty function a bit saner.

The previous coding supposed that the first differing bytes in two varlena
datums must have the same sign difference as their overall comparison
result.  This is obviously bogus for text strings in non-C locales, and
probably wrong for numeric, and even for bytea I think it was wrong on
machines where char is signed.  When the assumption failed, the function
could deliver a zero or negative penalty in situations where such a result
is quite ridiculous, leading the core GiST code to make very bad page-split
decisions.

To fix, take the absolute values of the byte-level differences.  Also,
switch the code to using unsigned char not just char, so that the behavior
will be consistent whether char is signed or not.

Per investigation of a trouble report from Tomas Vondra.  Back-patch to all
supported branches.
Fix performance issue in EXPLAIN (ANALYZE, TIMING OFF).

Commit af7914c6627bcf0b0ca614e9ce95d3f8056602bf, which added the TIMING
option to EXPLAIN, had an oversight: if the TIMING option is disabled
then control in InstrStartNode() goes through an elog(DEBUG2) call, which
typically does nothing but takes a noticeable amount of time to do it.
Tweak the logic to avoid that.

In HEAD, also change the elog(DEBUG2)'s in instrument.c to elog(ERROR).
It's not very clear why they weren't like that to begin with, but this
episode shows that not complaining more vociferously about misuse is
likely to do little except allow bugs to remain hidden.

While at it, adjust some code that was making possibly-dangerous
assumptions about flag bits being in the rightmost byte of the
instrument_options word.

Problem reported by Pavel Stehule (via Tomas Vondra).
doc: Rewrite how to get the source code

Instead of hardcoding a specific link, give a general link to the
download section of the web site.  This gives the user more download
options and the sysadmins more flexibility.  Also, the previously
presented link didn't work for devel versions.
Fix typo in comment

Etsuro Fujita
scripts: Add build prerequisite on libpgport

Without this, building in src/bin/scripts directly will fail if
libpgport wasn't built first.  Other bin components are handled the same
way.

Phil Sorber
Exclude access/rmgrlist.h from cpluspluscheck

It is not meant to be included standalone.
Fix another typo in a comment

Noted by Thom Brown
doc: Fix mistakes in the most recent set of release notes.

Improve description of the vacuum_freeze_table_age bug (it's much more
serious than we realized at the time the fix was committed), and correct
attribution of pg_upgrade -O/-o fix (Marti Raudsepp contributed that,
but Bruce forgot to credit him in the commit log).

No need to back-patch right now, it'll happen when the next set of
release notes are prepared.
Fix Xmax freeze conditions

I broke this in 0ac5ad5134; previously, freezing a tuple marked with an
IS_MULTI xmax was not necessary.

Per brokenness report from Jeff Janes.
Clean up c.h / postgres.h after Assert() move

Per Tom