Plug some more holes in encoding conversion.

Various places assume that pg_do_encoding_conversion() and
pg_server_to_any() will ensure encoding validity of their results;
but they failed to do so in the case that the source encoding is SQL_ASCII
while the destination is not.  We cannot perform any actual "conversion"
in that scenario, but we should still validate the string according to the
destination encoding.  Per bug #9210 from Digoal Zhou.

Arguably this is a back-patchable bug fix, but on the other hand adding
more enforcing of encoding checks might break existing applications that
were being sloppy.  On balance there doesn't seem to be much enthusiasm
for a back-patch, so fix in HEAD only.

While at it, remove some apparently-no-longer-needed provisions for
letting pg_do_encoding_conversion() "work" outside a transaction ---
if you consider it "working" to silently fail to do the requested
conversion.

Also, make a few cosmetic improvements in mbutils.c, notably removing
some Asserts that are certainly dead code since the variables they
assert aren't null are never null, even at process start.  (I think
this wasn't true at one time, but it is now.)
Prefer pg_any_to_server/pg_server_to_any over pg_do_encoding_conversion.

A large majority of the callers of pg_do_encoding_conversion were
specifying the database encoding as either source or target of the
conversion, meaning that we can use the less general functions
pg_any_to_server/pg_server_to_any instead.

The main advantage of using the latter functions is that they can make use
of a cached conversion-function lookup in the common case that the other
encoding is the current client_encoding.  It's notationally cleaner too in
most cases, not least because of the historical artifact that the latter
functions use "char *" rather than "unsigned char *" in their APIs.

Note that pg_any_to_server will apply an encoding verification step in
some cases where pg_do_encoding_conversion would have just done nothing.
This seems to me to be a good idea at most of these call sites, though
it partially negates the performance benefit.

Per discussion of bug #9210.
doc: Improve DocBook XML validity

DocBook XML is superficially compatible with DocBook SGML but has a
slightly stricter DTD that we have been violating in a few cases.
Although XSLT doesn't care whether the document is valid, the style
sheets don't necessarily process invalid documents correctly, so we need
to work toward fixing this.

This first commit moves the indexterms in refentry elements to an
allowed position.  It has no impact on the output.
Remove a couple of comments from the pg_lsn regression test.

Previously, one of these was a negative test case, but that got
changed along the way and the comments didn't get the memo.

Michael Paquier
Use pg_lsn data type in pg_stat_replication, too.

Michael Paquier, per a suggestion from Andres Freund
docs: document behavior of CHAR() comparisons with chars < space

Space trimming rather than space-padding causes unusual behavior, which
might not be standards-compliant.

Also remove recently-added now-redundant C comment.
Allow single-point polygons to be converted to circles

This allows finding the center of a single-point polygon and converting
it to a point.

Per report from Josef Grahn
Fix markup for CHAR() doc patch
pg_dump:  fix subtle memory leak in func and arg signature processing
psql:  add separate \d display for disabled system triggers

Previously if you disabled all triggers, only user triggers would
show as disabled

Per report from Andres Freund
docs:  remove unnecessary references to old PG versions
Increase work_mem and maintenance_work_mem defaults by 4x

New defaults are 4MB and 64MB.
Update and clarify ssl_ciphers default

- Write HIGH:MEDIUM instead of DEFAULT:!LOW:!EXP for clarity.
- Order 3DES last to work around inappropriate OpenSSL default.
- Remove !MD5 and @STRENGTH, because they are irrelevant.
- Add clarifying documentation.

Effectively, the new default is almost the same as the old one, but it
is arguably easier to understand and modify.

Author: Marko Kreen <markokr@gmail.com>
pg_basebackup: Skip only the *contents* of pg_replslot.

Include the directory itself.

Fujii Masao
Show xid and xmin in pg_stat_activity and pg_stat_replication.

Christian Kruse, reviewed by Andres Freund and myself, with further
minor adjustments by me.
Update a few comments to mention materialized views.

Etsuro Fujita
Use SnapshotDirty rather than an active snapshot to probe index endpoints.

If there are lots of uncommitted tuples at the end of the index range,
get_actual_variable_range() ends up fetching each one and doing an MVCC
visibility check on it, until it finally hits a visible tuple.  This is
bad enough in isolation, considering that we don't need an exact answer
only an approximate one.  But because the tuples are not yet committed,
each visibility check does a TransactionIdIsInProgress() test, which
involves scanning the ProcArray.  When multiple sessions do this
concurrently, the ensuing contention results in horrid performance loss.
20X overall throughput loss on not-too-complicated queries is easy to
demonstrate in the back branches (though someone's made it noticeably
less bad in HEAD).

We can dodge the problem fairly effectively by using SnapshotDirty rather
than a normal MVCC snapshot.  This will cause the index probe to take
uncommitted tuples as good, so that we incur only one tuple fetch and test
even if there are many such tuples.  The extent to which this degrades the
estimate is debatable: it's possible the result is actually a more accurate
prediction than before, if the endmost tuple has become committed by the
time we actually execute the query being planned.  In any case, it's not
very likely that it makes the estimate a lot worse.

SnapshotDirty will still reject tuples that are known committed dead, so
we won't give bogus answers if an invalid outlier has been deleted but not
yet vacuumed from the index.  (Because btrees know how to mark such tuples
dead in the index, we shouldn't have a big performance problem in the case
that there are many of them at the end of the range.)  This consideration
motivates not using SnapshotAny, which was also considered as a fix.

Note: the back branches were using SnapshotNow instead of an MVCC snapshot,
but the problem and solution are the same.

Per performance complaints from Bartlomiej Romanski, Josh Berkus, and
others.  Back-patch to 9.0, where the issue was introduced (by commit
40608e7f949fb7e4025c0ddd5be01939adc79eec).
Fix crash in json_to_record().

json_to_record() depends on get_call_result_type() for the tuple
descriptor of the record that should be returned, but in some cases
that cannot be determined. Add a guard to check if the tuple
descriptor has been properly resolved, similar to other callers of
get_call_result_type().

Also add guard for two other callers of get_call_result_type() in
jsonfuncs.c. Although json_to_record() is the only actual bug, it's a
good idea to follow convention.
btbuild no longer calls _bt_doinsert(), update comment.

Peter Geoghegan
Fix WAL replay of locking an updated tuple

We were resetting the tuple's HEAP_HOT_UPDATED flag as well as t_ctid on
WAL replay of a tuple-lock operation, which is incorrect when the tuple
is already updated.

Back-patch to 9.3.  The clearing of both header elements was there
previously, but since no update could be present on a tuple that was
being locked, it was harmless.

Bug reported by Peter Geoghegan and Greg Stark in
CAM3SWZTMQiCi5PV5OWHb+bYkUcnCk=O67w0cSswPvV7XfUcU5g@mail.gmail.com and
CAM-w4HPTOeMT4KP0OJK+mGgzgcTOtLRTvFZyvD0O4aH-7dxo3Q@mail.gmail.com
respectively; diagnosis by Andres Freund.