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