Add SHELL_EXIT_CODE to psql

Over in
https://www.postgresql.org/message-id/eaf326ad693e74eba068f33a7f518039@oss.nttdata.com
Justin
Pryzby suggested that psql might need the ability to capture the shell exit
code.

This is a POC patch that does that, but doesn't touch on the ON_ERROR_STOP
stuff.

I've added some very rudimentary tests, but haven't touched the
documentation, because I strongly suspect that someone will suggest a
better name for the variable.

But basically, it works like this

-- SHELL_EXIT_CODE is undefined
\echo :SHELL_EXIT_CODE
:SHELL_EXIT_CODE
-- bad \!
\! borp
sh: line 1: borp: command not found
\echo :SHELL_EXIT_CODE
32512
-- bad backtick
\set var `borp`
sh: line 1: borp: command not found
\echo :SHELL_EXIT_CODE
127
-- good \!
\! true
\echo :SHELL_EXIT_CODE
0
-- play with exit codes
\! exit 4
\echo :SHELL_EXIT_CODE
1024
\set var `exit 3`
\echo :SHELL_EXIT_CODE
3

Feedback welcome.