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

src/test/regress/sql/psql.sql

@@ -1022,7 +1022,7 @@ select \if false \\ (bogus \else \\ 42 \endif \\ forty_two;
 	\timing arg1
 	\unset arg1
 	\w arg1
-	\watch arg1
+	\watch arg1 arg2
 	\x arg1
 	-- \else here is eaten as part of OT_FILEPIPE argument
 	\w |/no/such/file \else