docs:  improve TABLE command by showing supported clauses

Initial patch by Colin 't Hart
doc: improve server's keepalive description

Use superior libpq keepalive description for the server-level
parameters.

Per report by Tatsuo Ishii and Marko Tiikkaja
pg_upgrade: document delete problems with tablespaces inside the cluster directory

Per report by Marc Mamin
docs:  small adjustements to recent SELECT and pg_upgrade improvements
pg_ctl:  improve handling of invalid data directory

Return '4' and report a meaningful error message when a non-existent or
invalid data directory is passed.  Previously, pg_ctl would just report
the server was not running.

Patch by me and Amit Kapila
Report from Peter Eisentraut
C comments: improve description of relfilenode uniqueness

Report by Antonin Houska
doc: remove extra whitespace in SGML markup
Correct copy/pasto in comment for REPLICA IDENTITY
Teach on_exit_reset() to discard pending cleanups for dsm.

If a postmaster child invokes fork() and then calls on_exit_reset, that
should be sufficient to let it exit() without breaking anything, but
dynamic shared memory broke that by not updating on_exit_reset() to
discard callbacks registered with dynamic shared memory segments.

Per investigation of a complaint from Tom Lane.
Allow logical decoding via the walsender interface.

In order for this to work, walsenders need the optional ability to
connect to a database, so the "replication" keyword now allows true
or false, for backward-compatibility, and the new value "database"
(which causes the "dbname" parameter to be respected).

walsender needs to loop not only when idle but also when sending
decoded data to the user and when waiting for more xlog data to decode.
This means that there are now three separate loops inside walsender.c;
although some refactoring has been done here, this is still a bit ugly.

Andres Freund, with contributions from Álvaro Herrera, and further
review by me.
Allow dynamic shared memory segments to be kept until shutdown.

Amit Kapila, reviewed by Kyotaro Horiguchi, with some further
changes by me.
Fix tracking of psql script line numbers during \copy from another place.

Commit 08146775acd8bfe0fcc509c71857abb928697171 changed do_copy() to
temporarily scribble on pset.cur_cmd_source.  That was a mighty ugly bit of
code in any case, but in particular it broke handleCopyIn's ability to tell
whether it was reading from the current script source file (in which case
pset.lineno should be incremented for each line of COPY data), or from
someplace else (in which case it shouldn't).  The former case still worked,
the latter not so much.  The visible effect was that line numbers reported
for errors in a script file would be wrong if there were an earlier \copy
that was reading anything other than inline-in-the-script-file data.

To fix, introduce another pset field that holds the file do_copy wants the
COPY code to use.  This is a little bit ugly, but less so than passing the
file down explicitly through several layers that aren't COPY-specific.

Extracted from a larger patch by Kumar Rajeev Rastogi; that patch also
changes printing of COPY command tags, which is not a bug fix and shouldn't
get back-patched.  This particular idea was from a suggestion by Amit
Khandekar, if I'm reading the thread correctly.

Back-patch to 9.2 where the faulty code was introduced.
add documentation for GetSpecialCustomVar method
add SupportCustomBackwardScan and FinalizeCustomPlan instead of
the backward_scan and custom_expr fields in CustomPlan.
remove backward_scan from CustomPlanState
Merge branch 'master' into custom_scan
cleanup create_custom_plan()
cosmetic change