pgsql: psql: add an optional execution-count limit to \watch.

psql: add an optional execution-count limit to \watch.

\watch can now be told to stop after N executions of the query.

With the idea that we might want to add more options to \watch
in future, this patch generalizes the command's syntax to a list
of name=value options, with the interval allowed to omit the name
for backwards compatibility.

Andrey Borodin, reviewed by Kyotaro Horiguchi, Nathan Bossart,
Michael Paquier, Yugo Nagata, and myself

Discussion: https://postgr.es/m/CAAhFRxiZ2-n_L1ErMm9AZjgmUK=qS6VHb+0SaMn8sqqbhF7How@mail.gmail.com

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/00beecfe839c878abb366b68272426ed5296bc2b

Modified Files
--------------
doc/src/sgml/ref/psql-ref.sgml     |  10 +++-
src/bin/psql/command.c             | 118 +++++++++++++++++++++++++++++++------
src/bin/psql/help.c                |   2 +-
src/bin/psql/t/001_basic.pl        |  33 ++++++++---
src/test/regress/expected/psql.out |   2 +-
src/test/regress/sql/psql.sql      |   2 +-
6 files changed, 135 insertions(+), 32 deletions(-)