docs: Clarify flag dependencies for background workers.

BGWORKER_BACKEND_DATABASE_CONNECTION can only be used if
BGWORKER_SHMEM_ACCESS is also used.

Michael Paquier, with some tweaks by me.
Add new GUC, max_worker_processes, limiting number of bgworkers.

In 9.3, there's no particular limit on the number of bgworkers;
instead, we just count up the number that are actually registered,
and use that to set MaxBackends.  However, that approach causes
problems for Hot Standby, which needs both MaxBackends and the
size of the lock table to be the same on the standby as on the
master, yet it may not be desirable to run the same bgworkers in
both places.  9.3 handles that by failing to notice the problem,
which will probably work fine in nearly all cases anyway, but is
not theoretically sound.

A further problem with simply counting the number of registered
workers is that new workers can't be registered without a
postmaster restart.  This is inconvenient for administrators,
since bouncing the postmaster causes an interruption of service.
Moreover, there are a number of applications for background
processes where, by necessity, the background process must be
started on the fly (e.g. parallel query).  While this patch
doesn't actually make it possible to register new background
workers after startup time, it's a necessary prerequisite.

Patch by me.  Review by Michael Paquier.
Add contrib function references in the doc index
Backpatch to 9.3.
Idea from Craig Ringer
Add C comment about \copy bug in CSV mode
Comment: This code erroneously assumes '\.' on a line alone inside a
quoted CSV string terminates the \copy.
http://www.postgresql.org/message-id/E1TdNVQ-0001ju-GO@wrigleys.postgresql.org
Fix typo in comment.

Michael Paquier
Use type "int64" for memory accounting in tuplesort.c/tuplestore.c.

Commit 263865a48973767ce8ed7b7788059a38a24a9f37 switched tuplesort.c and
tuplestore.c variables representing memory usage from type "long" to
type "Size".  This was unnecessary; I thought doing so avoided overflow
scenarios on 64-bit Windows, but guc.c already limited work_mem so as to
prevent the overflow.  It was also incomplete, not touching the logic
that assumed a signed data type.  Change the affected variables to
"int64".  This is perfect for 64-bit platforms, and it reduces the need
to contemplate platform-specific overflow scenarios.  It also puts us
close to being able to support work_mem over 2 GiB on 64-bit Windows.

Per report from Andres Freund.