Improve WIN32 waiting logic in psql's \watch command.

Improve WIN32 waiting logic in psql's \watch command.

do_watch had some leftover logic for enabling siglongjmp out of
waiting for input.  That's never done anything on Windows (cf.
psql_cancel_callback), and do_watch no longer relies on it for
non-Windows, so let's drop it.

Also, when the user cancels \watch by pressing ^C, the Windows
code would run the query one more time before exiting.  That doesn't
seem very desirable, and it's not what happens on other platforms.
Use the "done" flag similarly to non-Windows to avoid the extra query
execution.

Yugo Nagata (with minor fixes by me)

Discussion: https://postgr.es/m/20240305220552.85fd4afd6b6b8103bf4fe3d0@sraoss.co.jp