Do ScalarArrayOp estimation correctly when array is a stable expression.

Most estimation functions apply estimate_expression_value to see if they
can reduce an expression to a constant; the key difference is that it
allows evaluation of stable as well as immutable functions in hopes of
ending up with a simple Const node.  scalararraysel didn't get the memo
though, and neither did gincost_opexpr/gincost_scalararrayopexpr.  Fix
that, and remove a now-unnecessary estimate_expression_value step in the
subsidiary function scalararraysel_containment.

Per complaint from Alexey Klyukin.  Back-patch to 9.3.  The problem
goes back further, but I'm hesitant to change estimation behavior in
long-stable release branches.
pg_basebackup: Add support for relocating tablespaces

Tablespaces can be relocated in plain backup mode by specifying one or
more -T olddir=newdir options.

Author: Steeve Lennmark <steevel@handeldsbanken.se>
Reviewed-by: Peter Eisentraut <peter_e@gmx.net>
configure.in: Use dnl in place of # where appropriate

The comment added by ed011d9754fd4b76eac0eaa8c057fcfc0c302a6a used #,
which means it gets copied into configure, but it doesn't make sense
there.  So use dnl, which gets dropped when creating configure.
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.