pgsql: Add new psql help topics, accessible to both --help and \?.

Add new psql help topics, accessible to both --help and \?.

Add --help=<topic> for the commandline, and \? <topic> as a backslash
command, to show more help than the invocations without parameters
do. "commands", "variables" and "options" currently exist as help
topics describing, respectively, backslash commands, psql variables,
and commandline switches. Without parameters the help commands show
their previous topic.

Some further wordsmithing or extending of the added help content might
be needed; but there seems little benefit delaying the overall feature
further.

Author: Pavel Stehule, editorialized by many

Reviewed-By: Andres Freund, Petr Jelinek, Fujii Masao, MauMau, Abhijit
Menon-Sen and Erik Rijkers.

Discussion: CAFj8pRDVGuC-nXBfe2CK8vpyzd2Dsr9GVpbrATAnZO=2YQ0s2Q(at)mail(dot)gmail(dot)com,
CAFj8pRA54AbTv2RXDTRxiAd8hy8wxmoVLqhJDRCwEnhdd7OUkw(at)mail(dot)gmail(dot)com

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/07c8651dd91d5aea38bee268acb582674ca4b5ea

Modified Files
--------------
doc/src/sgml/ref/psql-ref.sgml | 23 ++++-
src/bin/psql/command.c | 14 ++-
src/bin/psql/help.c | 210 +++++++++++++++++++++++++++++++---------
src/bin/psql/help.h | 4 +-
src/bin/psql/startup.c | 29 ++++--
src/bin/psql/tab-complete.c | 7 ++
6 files changed, 224 insertions(+), 63 deletions(-)