Fix typo in comment.

Also line-wrap an over-wide line in a comment that's ignored by pgindent.
Fix thinko in worker_spi, count(*) returns a bigint. Thanks RhodiumToad
docs: Clarify that we also support Solaris versions greater than 10.

MauMau
Docs:  add paragraph about date/timestamp subtraction

per suggestion from Francisco Olart
doc:  Fix SGML markup for date patch
Docs: wording improvements in discussion of timestamp arithmetic.

I started out just to fix the broken markup in commit
1c2085766187031eaeaae7db4785b9e1d4241988, but got distracted by
copy-editing.  I see Bruce already fixed the markup, but I'll
commit the wordsmithing anyway.
Update obsolete comment
Allow aggregate functions to be VARIADIC.

There's no inherent reason why an aggregate function can't be variadic
(even VARIADIC ANY) if its transition function can handle the case.
Indeed, this patch to add the feature touches none of the planner or
executor, and little of the parser; the main missing stuff was DDL and
pg_dump support.

It is true that variadic aggregates can create the same sort of ambiguity
about parameters versus ORDER BY keys that was complained of when we
(briefly) had both one- and two-argument forms of string_agg().  However,
the policy formed in response to that discussion only said that we'd not
create any built-in aggregates with varying numbers of arguments, not that
we shouldn't allow users to do it.  So the logical extension of that is
we can allow users to make variadic aggregates as long as we're wary about
shipping any such in core.

In passing, this patch allows aggregate function arguments to be named, to
the extent of remembering the names in pg_proc and dumping them in pg_dump.
You can't yet call an aggregate using named-parameter notation.  That seems
like a likely future extension, but it'll take some work, and it's not what
this patch is really about.  Likewise, there's still some work needed to
make window functions handle VARIADIC fully, but I left that for another
day.

initdb forced because of new aggvariadic field in Aggref parse nodes.
Don't fail for bad GUCs in CREATE FUNCTION with check_function_bodies off.

The previous coding attempted to activate all the GUC settings specified
in SET clauses, so that the function validator could operate in the GUC
environment expected by the function body.  However, this is problematic
when restoring a dump, since the SET clauses might refer to database
objects that don't exist yet.  We already have the parameter
check_function_bodies that's meant to prevent forward references in
function definitions from breaking dumps, so let's change CREATE FUNCTION
to not install the SET values if check_function_bodies is off.

Authors of function validators were already advised not to make any
"context sensitive" checks when check_function_bodies is off, if indeed
they're checking anything at all in that mode.  But extend the
documentation to point out the GUC issue in particular.

(Note that we still check the SET clauses to some extent; the behavior
with !check_function_bodies is now approximately equivalent to what ALTER
DATABASE/ROLE have been doing for awhile with context-dependent GUCs.)

This problem can be demonstrated in all active branches, so back-patch
all the way.
Update comments concerning PGC_S_TEST.

This GUC context value was once only used by ALTER DATABASE SET and
ALTER USER SET.  That's not true anymore, though, so rewrite the
comments to be a bit more general.

Patch in HEAD only, since this is just an internal documentation issue.