proposal - psql - use pager for \watch command

Hi,

last week I finished pspg 3.0 https://github.com/okbob/pspg . pspg now
supports pipes, named pipes very well. Today the pspg can be used as pager
for output of \watch command. Sure, psql needs attached patch.

I propose new psql environment variable PSQL_WATCH_PAGER. When this
variable is not empty, then \watch command starts specified pager, and
redirect output to related pipe. When pipe is closed - by pager, then
\watch cycle is leaved.

If you want to test proposed feature, you need a pspg with
cb4114f98318344d162a84b895a3b7f8badec241
commit.

Then you can set your env

export PSQL_WATCH_PAGER="pspg --stream"
psql

SELECT * FROM pg_stat_database;
\watch 1

Comments, notes?

Regards

Pavel