pgsql: Allow psql's \g and \gx commands to transiently change \pset opt

Allow psql's \g and \gx commands to transiently change \pset options.

We invented \gx to allow the "\pset expanded" flag to be forced on
for the duration of one command output, but that turns out to not
be nearly enough to satisfy the demand for variant output formats.
Hence, make it possible to change any pset option(s) for the duration
of a single command output, by writing "option=value ..." inside
parentheses, for example
        \g (format=csv csv_fieldsep='\t') somefile

\gx can now be understood as a shorthand for including expanded=on
inside the parentheses.

Patch by me, expanding on a proposal by Pavel Stehule

Discussion: https://postgr.es/m/CAFj8pRBx9OnBPRJVtfA5ycUpySge-XootAXAsv_4rrkHxJ8eRg@mail.gmail.com

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/b63c293bcbd7439f883cd4cf748f6755df0fbb3c

Modified Files
--------------
doc/src/sgml/ref/psql-ref.sgml     |  76 ++++++++++---
src/bin/psql/command.c             | 218 ++++++++++++++++++++++++++++++++++---
src/bin/psql/command.h             |   4 +
src/bin/psql/common.c              |  21 ++--
src/bin/psql/help.c                |   9 +-
src/bin/psql/settings.h            |   5 +-
src/test/regress/expected/psql.out |  22 ++++
src/test/regress/sql/psql.sql      |   7 ++
8 files changed, 315 insertions(+), 47 deletions(-)