The original coding for ALTER SYSTEM made a fundamentally bogus assumption
that postgresql.auto.conf could be sought relative to the main config file
if we hadn't yet determined the value of data_directory.  This fails for
common arrangements with the config file elsewhere, as reported by
Christoph Berg.

The simplest fix is to not try to read postgresql.auto.conf until after
SelectConfigFiles has chosen (and locked down) the data_directory setting.

Because of the logic in ProcessConfigFile for handling resetting of GUCs
that've been removed from the config file, we cannot easily read the main
and auto config files separately; so this patch adopts a brute force
approach of reading the main config file twice during postmaster startup.
That's a tad ugly, but the actual time cost is likely to be negligible,
and there's no time for a more invasive redesign before beta.

With this patch, any attempt to set data_directory via ALTER SYSTEM
will be silently ignored.  It would probably be better to throw an
error, but that can be dealt with later.  This bug, however, would
prevent any testing of ALTER SYSTEM by a significant fraction of the
userbase, so it seems important to get it fixed before beta.

There's no longer much pressure to switch the default GIN opclass for
jsonb, but there was still some unhappiness with the name "jsonb_hash_ops",
since hashing is no longer a distinguishing property of that opclass,
and anyway it seems like a relatively minor detail.  At the suggestion of
Heikki Linnakangas, we'll use "jsonb_path_ops" instead; that captures the
important characteristic that each index entry depends on the entire path
from the document root to the indexed value.

Also add a user-facing explanation of the implementation properties of
these two opclasses.

Per discussion, this seems like a more consistent choice of name.

Fabrízio de Royes Mello, after a suggestion by Peter Eisentraut;
some additional documentation wordsmithing by me

Document existence operator adequately; fix obsolete claim that no
Unicode-escape semantic checks happen on input (it's still true for
json, but not for jsonb); improve examples; assorted wordsmithing.

When returning rows from a bitmap, as done with partial match queries, we
would get stuck in an infinite loop if the bitmap contained a lossy page
reference.

This bug is new in master, it was introduced by the patch to allow skipping
items refuted by other entries in GIN scans.

Report and fix by Alexander Korotkov

Usually the search would find plain "tclsh" without any trouble,
but some installations might only have the version-numbered flavor
of that program.

No compatibility problems have been reported with 8.6, so we might
as well back-patch this to all active branches.

Christoph Berg