psql: Make default \watch interval configurable

psql: Make default \watch interval configurable

The default interval for \watch to wait between executing queries,
when executed without a specified interval, was hardcoded to two
seconds.  This adds the new variable WATCH_INTERVAL which is used
to set the default interval, making it configurable for the user.
This makes \watch the first command which has a user configurable
default setting.

Author: Daniel Gustafsson <[email protected]>
Reviewed-by: Heikki Linnakangas <[email protected]>
Reviewed-by: Michael Paquier <[email protected]>
Reviewed-by: Kirill Reshke <[email protected]>
Reviewed-by: Masahiro Ikeda <[email protected]>
Reviewed-by: Laurenz Albe <[email protected]>
Reviewed-by: Greg Sabino Mullane <[email protected]>
Reviewed-by: Ashutosh Bapat <[email protected]>
Discussion: https://postgr.es/m/B2FD26B4-8F64-4552-A603-5CC3DF1C7103@yesql.se