Add ALTER SYSTEM command to edit the server configuration file.

Patch contributed by Amit Kapila. Reviewed by Hari Babu, Masao Fujii,
Boszormenyi Zoltan, Andres Freund, Greg Smith and others.
Don't ignore tuple locks propagated by our updates

If a tuple was locked by transaction A, and transaction B updated it,
the new version of the tuple created by B would be locked by A, yet
visible only to B; due to an oversight in HeapTupleSatisfiesUpdate, the
lock held by A wouldn't get checked if transaction B later deleted (or
key-updated) the new version of the tuple.  This might cause referential
integrity checks to give false positives (that is, allow deletes that
should have been rejected).

This is an easy oversight to have made, because prior to improved tuple
locks in commit 0ac5ad5134f it wasn't possible to have tuples created by
our own transaction that were also locked by remote transactions, and so
locks weren't even considered in that code path.

It is recommended that foreign keys be rechecked manually in bulk after
installing this update, in case some referenced rows are missing with
some referencing row remaining.

Per bug reported by Daniel Wood in
CAPweHKe5QQ1747X2c0tA=5zf4YnS2xcvGf13Opd-1Mq24rF1cQ@mail.gmail.com
Fix incorrect error message reported for non-existent users

Previously, lookups of non-existent user names could return "Success";
it will now return "User does not exist" by resetting errno.  This also
centralizes the user name lookup code in libpgport.

Report and analysis by Nicolas Marchildon;  patch by me
Allow on-detach callbacks for dynamic shared memory segments.

Just as backends must clean up their shared memory state (releasing
lwlocks, buffer pins, etc.) before exiting, they must also perform
any similar cleanups related to dynamic shared memory segments they
have mapped before unmapping those segments.  So add a mechanism to
ensure that.

Existing on_shmem_exit hooks include both "user level" cleanup such
as transaction abort and removal of leftover temporary relations and
also "low level" cleanup that forcibly released leftover shared
memory resources.  On-detach callbacks should run after the first
group but before the second group, so create a new before_shmem_exit
function for registering the early callbacks and keep on_shmem_exit
for the regular callbacks.  (An earlier draft of this patch added an
additional argument to on_shmem_exit, but that had a much larger
footprint and probably a substantially higher risk of breaking third
party code for no real gain.)

Patch by me, reviewed by KaiGai Kohei and Andres Freund.
Fix compiler warning.

get_user_name returns const char *, but we were assigning the result
to a char * variable.
Upgrade to Autoconf 2.69
Fix typo in docs for min_recovery_apply_delay.

Bernd Helmle
Add tab completion for ALTER SYSTEM SET in psql.
Optimize updating a row that's locked by same xid

Updating or locking a row that was already locked by the same
transaction under the same Xid caused a MultiXact to be created; but
this is unnecessary, because there's no usefulness in being able to
differentiate two locks by the same transaction.  In particular, if a
transaction executed SELECT FOR UPDATE followed by an UPDATE that didn't
modify columns of the key, we would dutifully represent the resulting
combination as a multixact -- even though a single key-update is
sufficient.

Optimize the case so that only the strongest of both locks/updates is
represented in Xmax.  This can save some Xmax's from becoming
MultiXacts, which can be a significant optimization.

This missed optimization opportunity was spotted by Andres Freund while
investigating a bug reported by Oliver Seemann in message
CANCipfpfzoYnOz5jj=UZ70_R=CwDHv36dqWSpwsi27vpm1z5sA@mail.gmail.com
and also directly as a performance regression reported by Dong Ye in
message
d54b8387.000012d8.00000010@YED-DEVD1.vmware.com
Reportedly, this patch fixes the performance regression.

Since the missing optimization was reported as a significant performance
regression from 9.2, backpatch to 9.3.

Andres Freund, tweaked by Álvaro Herrera
Make stdout unbuffered

This ensures that all stdout output is flushed immediately, to match
stderr.  This eliminates the need for fflush(stdout) calls sprinkled all
over the place.

Per Daniel Wood in message 519A79C6.90308@salesforce.com
Move pg_upgrade_support global variables to their own include file

Previously their declarations were spread around to avoid accidental
access.
isolationtester: Ensure stderr is unbuffered, too
Make various variables read-only (const)

This allows the variables to be moved from .data to .rodata section which
means that more data can be shared by processes and makes sure that nothing
can accidentally overwrite the read-only definitions.  On a x86-64 Linux
system this moves roughly 9 kilobytes of previously writable data to the
read-only data segment in the backend and 4 kilobytes in libpq.